Add scalar floor and refactor temperature and density floors

[helenarichie]

In summary, this PR:

• Adds the ability to apply a floor to any scalar in the conserved array in Apply_Scalar_Floor.
  • The particular scalar the floor is applied to is specified using the field_num argument of Apply_Scalar_Floor, and should be specified with its grid_enum index.
• Refactors the density, temperature, and scalar floor values to be set from the input parameter file.
  • Deletes the DENS_FLOOR and TEMP_FLOOR macros (the floors are instead stored in the Grid3D header as density_floor, temperature_floor, and scalar_floor).
• Adds a unit test for Apply_Scalar_Floor.
• Refactors where the scalar and temperature floor functions are called from based on Issue Move location of temperature floor #362.

[evaneschneider]

While I understand this logic for the density and temperature floors, I think it doesn't actually apply for the scalar floor, since scalars can have negative values (think for example about the transverse velocities in the Riemann problem). In practice, it shouldn't matter, since you don't call the floor function unless you are applying a floor, in which case the value should have been specified in the parameter file. But I think it might make more sense rather than setting a default value of 0 here to just have the code throw a warning if the floor macro is turned on but no input value is supplied.

[evaneschneider]

New parameter defaults should be set in global.h, and the code should give a warning if any floor is turned on but the input parameter isn't set.

[helenarichie]

New parameter defaults should be set in global.h, and the code should give a warning if any floor is turned on but the input parameter isn't set.

Addressed in 3e1c195