P-level interpolator is Earth-centric

[sit23]

The script that interpolates data from sigma levels to pressure levels, Isca/postprocessing/plevel_interpolation/src/postprocessing/plevel/run_pressure_interp.F90 gets its gravitational constants, etc, from plev_constants.F90 in that same folder. These are parameters and are set to Earth values. For proper interpolation in planetary applications, these values should be made to be input parameters.