Clean up linked list code (following review)

1. Typo in marbl_interface.F90 -- abort if registered_output is not associated
   (was checking this%output_for_gcm_registry%registered_outputs by accident)
2. Add comment about why create_registry() resets err_message to "" before each
   entry (and clean up errant white-space misalignment)
3. Also, blocks in create_registry() are easier to read if we set err_message
   to "" and then use a continutation line for the if () rather than using
   if-else
4. For adding to the registry linked list, order doesn't matter but the logic
   is simpler (and code is faster) if we prepend entries rather than looping
   through to the end of the list every time

src/marbl_interface.F90

@@ -815,7 +815,7 @@ subroutine add_output_for_GCM(this, num_elements, field_name, output_id, field_s
  end do
 
  ! Abort if field_name was not registered
- if (.not. associated(this%output_for_gcm_registry%registered_outputs)) then
+ if (.not. associated(registered_output)) then
  write(log_message, "(2A)") trim(field_name), " is not a valid output field name for the GCM"
  call this%StatusLog%log_error(log_message, subname)
  return

src/marbl_interface_public_types.F90

@@ -797,13 +797,16 @@ subroutine create_registry(this, conc_flux_units)
 
  character(len=char_len) :: err_message
 
+ ! NOTE: we want to set err_message = "" prior to every conditional
+ ! so we don't inadvertently pass a non-empty string from a
+ ! previous output variable
+ ! E.g. if lflux_gas_o2 = F but lflux_gas_co2 = T, err_message
+ ! should be blank for CO2 flux (assuming base_bio_on = T)
 
  ! Oxygen Flux
- if (.not. (base_bio_on .and. lflux_gas_o2)) then
+ err_message = ""
+ if (.not. (base_bio_on .and. lflux_gas_o2)) &
  err_message = "requires base biotic tracers and lflux_gas_o2"
- else
- err_message = ""
- end if
  call this%add_registry_entry(short_name = "flux_o2", &
  long_name = "Oxygen Flux", &
  units = conc_flux_units, &
@@ -812,56 +815,48 @@ subroutine create_registry(this, conc_flux_units)
  err_message = err_message)
 
  ! Carbon Dioxide Flux
- if (.not. (base_bio_on .and. lflux_gas_co2)) then
+ err_message = ""
+ if (.not. (base_bio_on .and. lflux_gas_co2)) &
  err_message = "requires base biotic tracers and lflux_gas_co2"
- else
- err_message = ""
- end if
  call this%add_registry_entry(short_name = "flux_co2", &
-  long_name = "Carbon Dioxide Flux", &
-  units = conc_flux_units, &
-  field_source = "surface_flux", &
-  id = ofg_ind%flux_co2_id, &
-  err_message = err_message)
+ long_name = "Carbon Dioxide Flux", &
+ units = conc_flux_units, &
+ field_source = "surface_flux", &
+ id = ofg_ind%flux_co2_id, &
+ err_message = err_message)
 
  ! NHx Surface Emissions
- if (.not. (base_bio_on .and. lcompute_nhx_surface_emis)) then
+ err_message = ""
+ if (.not. (base_bio_on .and. lcompute_nhx_surface_emis)) &
  err_message = "requires base biotic tracers and lcompute_nhx_surface_emis"
- else
- err_message = ""
- end if
  call this%add_registry_entry(short_name = "flux_nhx", &
-  long_name = "NHx Surface Emissions", &
-  units = conc_flux_units, &
-  field_source = "surface_flux", &
-  id = ofg_ind%flux_nhx_id, &
-  err_message = err_message)
+ long_name = "NHx Surface Emissions", &
+ units = conc_flux_units, &
+ field_source = "surface_flux", &
+ id = ofg_ind%flux_nhx_id, &
+ err_message = err_message)
 
  ! Surface Chlorophyll
- if (.not. base_bio_on) then
+ err_message = ""
+ if (.not. base_bio_on) &
  err_message = "requires base biotic tracers"
- else
- err_message = ""
- end if
  call this%add_registry_entry(short_name = "total_surfChl", &
-  long_name = "Total Surface Chlorophyll Concentration", &
-  units = "mg/m^3", &
-  field_source = "surface_flux", &
-  id = ofg_ind%total_surfChl_id, &
-  err_message = err_message)
+ long_name = "Total Surface Chlorophyll Concentration", &
+ units = "mg/m^3", &
+ field_source = "surface_flux", &
+ id = ofg_ind%total_surfChl_id, &
+ err_message = err_message)
 
  ! Full Depth Chlorophyll
- if (.not. base_bio_on) then
+ err_message = ""
+ if (.not. base_bio_on) &
  err_message = "requires base biotic tracers"
- else
- err_message = ""
- end if
  call this%add_registry_entry(short_name = "total_Chl", &
-  long_name = "Total Chlorophyll Concentration", &
-  units = "mg/m^3", &
-  field_source = "interior_tendency", &
-  id = ofg_ind%total_Chl_id, &
-  err_message = err_message)
+ long_name = "Total Chlorophyll Concentration", &
+ units = "mg/m^3", &
+ field_source = "interior_tendency", &
+ id = ofg_ind%total_Chl_id, &
+ err_message = err_message)
 
  end subroutine create_registry
 
@@ -878,19 +873,10 @@ subroutine add_registry_entry(this, short_name, long_name, units, field_source,
 
  type(marbl_output_for_GCM_linked_list_type), pointer :: new_entry
 
- ! Find last linked list item
- if (associated(this%registered_outputs)) then
- new_entry => this%registered_outputs
- do while (associated(new_entry%next))
- new_entry => new_entry%next
- end do
- allocate(new_entry%next)
- new_entry => new_entry%next
- else
- ! This is first entry in the registry
- allocate(new_entry)
- this%registered_outputs => new_entry
- end if
+ ! Insert new entry at beginning of linked list
+ allocate(new_entry)
+ new_entry%next => this%registered_outputs
+ this%registered_outputs => new_entry
 
  new_entry%short_name = short_name
  new_entry%long_name = long_name