Introduce gr_initialize_field_data

This change was split off from GH PR grackle-project#197. This should be a lot easier
to review in isolation (and I think that it is a generally useful
change in its own right).

Overview
--------
The objective of this PR is conceptually very simple. It introduces the
following function:

```c++
int gr_initialize_field_data(grackle_field_data *my_fields);
```

The idea is to have users immediately call this function right after they
initialize a new `grackle_field_data` instance

- When it's called the handful of non data-field members are assigned
  sensible defaults. It also initializes all data-field members to NULL
  pointers.

- this is analogous to the way that `local_initialize_chemistry_parameters`
  is used.

Motivation
----------

While this function is not strictly necessary, I think it is a useful
addition and I think we should tell people to use it (though old code
won't break).

A large motivation is peace of mind. I have often wished for this sort
of thing. I'm always a little concerned when using a new configuration
of grackle that I didn't initialize all of the required fields. I
would definitely feel a little more comfortable knowing that a field
that I forgot about is assigned a ``NULL`` pointer rather than some
garbage data that might let the code chug along.

It could also facillitate more error checking:

- such as checks in the style of the ``grid_dx`` check introduced in
  PR grackle-project#190 (applying similar checks to `grid_rank`, `grid_dimension`,
  `grid_start`, and `grid_end` requires that we know their default
  value).

- we could explicitly check that the user specified all required
  data-fields. We can only do this if we know that unset data-fields
  have a default value of `NULL`.

  - **From a user friendliness perspective, I think this check alone is
    enough to justify the function's existence.**

  - We could also potentially warn if unnecessary fields were specified

  - Plus, we could account for the fact that functions like
    ``calculate_pressure`` require fewer fields that ``solve_chemistry``

- we could also add checks that none of the fields are aliases (an
  implicit Fortran requirement). This would be much easier to implement
  if we knew we could just ignore fields that were set to ``NULL``
  pointers

We would probably need to make these checks opt-in somehow, both because
we can't be absolutely certain whether a user actually invoked
``gr_initialize_field_data`` and users might not want the runtime-cost
(which could be relatively large for very small grids)

Naming
------

I choose to name this function based on my suggestion in grackle-project#189 that we
adopt a convention that all new functions in the stable public API share
a common prefix (`gr_` or `grackle_`).

For the sake of proposing something concrete, I assumed that we choose
the `gr_` prefix. (If we decide on a different prefix, this would be
easy to change).

Alternatively, if we don't want any prefix, we could name it
`initialize_grackle_field_data` (I do worry a little that could
introduce naming conflicts with existing functions in downstream codes)

doc/source/Integration.rst

@@ -604,6 +604,9 @@ not intend to use.
   // Create struct for storing grackle field data
   grackle_field_data my_fields;
 
+  // initialize members of my_fields to sensible defaults
+  gr_initialize_field_data(&my_fields);
+
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.
   int field_size = 1;

doc/source/Reference.rst

@@ -13,6 +13,8 @@ Grackle has two versions of most functions.
    and :c:data:`chemistry_data_storage` instances to be provided as
    arguments. These are explicity thread-safe as they use no global data.
 
+Additionally, there are also some :ref:`misc_functions` and :ref:`rate-functions`.
+
 .. _primary_functions:
 
 Primary Functions
@@ -417,3 +419,20 @@ The following functions are used to query the name of the ith field of the :c:da
    :param unsigned int i: The index of the accessed parameter
    :rtype: const char*
    :returns: Pointer to the string-literal specifying the name. This is ``NULL``, if :c:data:`chemistry_data` has ``i`` or fewer ``string`` members.
+
+.. _misc_functions:
+
+Miscellaneous Functions
+-----------------------
+
+.. c:function:: int gr_initialize_field_data(grackle_field_data *my_fields);
+
+   Initializes the struct-members stored in the :c:type:`grackle_field_data` data structure to their default values.
+
+   This function must assume that any existing data within the data structure is garbage data.
+   In other words, when this function goes to overwrite a given member of :c:type:`grackle_field_data`, it completely ignores the value currently held by the member (i.e. the function does not provide special handling for members holding non- ``NULL`` pointers).
+   Consequently, this function should **ONLY** be called **BEFORE** initializing any members of the data structure (if it's called at any other time, the program may leak memory resources).
+
+   :param grackle_field_data \*my_fields: uninitialized field data storage
+   :rtype: int
+   :returns: 1 (success) or 0 (failure)

src/clib/grackle.h

@@ -117,4 +117,6 @@ int local_free_chemistry_data(chemistry_data *my_chemistry, chemistry_data_stora
 
 grackle_version get_grackle_version(void);
 
+int gr_initialize_field_data(grackle_field_data *my_fields);
+
 #endif

src/clib/grackle_field_data_fdatamembers.def

@@ -0,0 +1,57 @@
+/***********************************************************************
+/
+/ this file lists each member of the grackle_field_data struct that is
+/ intended to hold field data. This list is intended to be used with
+/ X-Macros in *.c files (to reduce the amount of code required to
+/ interact with these fields)
+/
+/ The following information is specified for each field:
+/    1. the field_name
+/    2. a 1 or 0 to denote whether the field_name corresponds to a
+/       passive-scalar density field
+/
+/
+/ Copyright (c) 2013, Enzo/Grackle Development Team.
+/
+/ Distributed under the terms of the Enzo Public Licence.
+/
+/ The full license is in the file LICENSE, distributed with this
+/ software.
+************************************************************************/
+
+ENTRY(density, 0)
+ENTRY(HI_density, 1)
+ENTRY(HII_density, 1)
+ENTRY(HM_density, 1)
+ENTRY(HeI_density, 1)
+ENTRY(HeII_density, 1)
+ENTRY(HeIII_density, 1)
+ENTRY(H2I_density, 1)
+ENTRY(H2II_density, 1)
+ENTRY(DI_density, 1)
+ENTRY(DII_density, 1)
+ENTRY(HDI_density, 1)
+ENTRY(e_density, 1)
+ENTRY(metal_density, 1)
+ENTRY(dust_density, 1)
+
+ENTRY(internal_energy, 0)
+ENTRY(x_velocity, 0)
+ENTRY(y_velocity, 0)
+ENTRY(z_velocity, 0)
+
+ENTRY(volumetric_heating_rate, 0)
+ENTRY(specific_heating_rate, 0)
+
+ENTRY(temperature_floor, 0)
+
+ENTRY(RT_heating_rate, 0)
+ENTRY(RT_HI_ionization_rate, 0)
+ENTRY(RT_HeI_ionization_rate, 0)
+ENTRY(RT_HeII_ionization_rate, 0)
+ENTRY(RT_H2_dissociation_rate, 0)
+
+ENTRY(H2_self_shielding_length, 0)
+ENTRY(H2_custom_shielding_factor, 0)
+
+ENTRY(isrf_habing, 0)

src/clib/grackle_fortran_interface.def

@@ -245,3 +245,11 @@ c     The following define the fortran interfaces to the C routines
             REAL(C_DOUBLE), INTENT(OUT) :: dust_temperature(*)
          END FUNCTION calculate_dust_temperature
       END INTERFACE
+
+      INTERFACE
+         INTEGER(C_INT) FUNCTION gr_initialize_field_data(my_fields)
+     &        bind(C)
+            IMPORT
+            TYPE(grackle_field_data), INTENT(INOUT) :: my_fields
+         END FUNCTION gr_initialize_field_data
+      END INTERFACE

src/clib/set_default_chemistry_parameters.c

@@ -41,3 +41,25 @@ int set_default_chemistry_parameters(chemistry_data *my_grackle)
   grackle_data = my_grackle;
   return local_initialize_chemistry_parameters(my_grackle);
 }
+
+int gr_initialize_field_data(grackle_field_data *my_fields)
+{
+  if (my_fields == NULL) {
+    fprintf(stderr, "gr_initial_field_data was passed a NULL pointer\n");
+    return FAIL;
+  }
+
+  my_fields->grid_rank = -1;
+  my_fields->grid_dimension = NULL;
+  my_fields->grid_start = NULL;
+  my_fields->grid_end = NULL;
+  my_fields->grid_dx = -1.0;
+
+  // now, modify all members holding datafields to have values of NULL
+  // (we use X-Macros to do this)
+  #define ENTRY(MEMBER_NAME, _1) my_fields->MEMBER_NAME = NULL;
+  #include "grackle_field_data_fdatamembers.def"
+  #undef ENTRY
+
+  return SUCCESS;
+}

src/example/c_example.c

@@ -77,6 +77,7 @@ int main(int argc, char *argv[])
 
   // Create struct for storing grackle field data
   grackle_field_data my_fields;
+  gr_initialize_field_data(&my_fields);
 
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.
@@ -244,4 +245,4 @@ int main(int argc, char *argv[])
   fprintf(stderr, "dust_temperature = %g K.\n", dust_temperature[0]);
 
   return EXIT_SUCCESS;
-}
+}

src/example/c_local_example.c

@@ -78,6 +78,7 @@ int main(int argc, char *argv[])
 
   // Create struct for storing grackle field data
   grackle_field_data my_fields;
+  gr_initialize_field_data(&my_fields);
 
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.

src/example/cxx_grid_example.C

@@ -76,6 +76,7 @@ grackle_field_data construct_field_data(grid_props& my_grid_props,
 
   // Create struct for storing grackle field data
   grackle_field_data my_fields;
+  gr_initialize_field_data(&my_fields);
 
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.

src/example/cxx_omp_example.C

@@ -181,6 +181,8 @@ int main(int argc, char *argv[])
   }
 
   grackle_field_data my_fields_t1, my_fields_tN;
+  gr_initialize_field_data(&my_fields_t1);
+  gr_initialize_field_data(&my_fields_tN);
 
   // Set grid dimension and size.
   // grid_start and grid_end are used to ignore ghost zones.

src/example/fortran_example.F

@@ -121,6 +121,7 @@ program fortran_example
       iresult = initialize_chemistry_data(my_units)
 
 c     Set field arrays
+      iresult = gr_initialize_field_data(my_fields)
 
 c     If grid rank is less than 3, set the other dimensions, 
 c     start indices, and end indices to 0.

src/python/pygrackle/grackle_defs.pxd

@@ -249,3 +249,5 @@ cdef extern from "grackle.h":
         c_chemistry_data_storage *my_rates)
 
     c_grackle_version c_get_grackle_version "get_grackle_version"()
+
+    int gr_initialize_field_data(c_field_data *my_fields)

src/python/pygrackle/grackle_wrapper.pyx

@@ -696,6 +696,7 @@ cdef c_field_data setup_field_data(object fc, int[::1] buf,
 
     # now initialize my_fields
     cdef c_field_data my_fields
+    gr_initialize_field_data(&my_fields)
     my_fields.grid_rank = 1
     my_fields.grid_dimension = grid_dimension
     my_fields.grid_start = grid_start