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 #362.

[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