Unify grib and netcdf sections of code where possible in ProcSat_interpolation.py

[JessicaMeixner-NOAA]

There are two main functions in ProcSat_interpolation.py depending on if the input is grib or netcdf. There has been an effort already to get these two functions to be more similar, however, there is still quite a bit of duplicate code. In an effort to help with future bug-fixes and general maintenance of the code, these two functions should be unified as much as possible and only having two parts of the code for the reading of the inputs. To achieve this, some of the inputs will have to be renamed after read in so that there is consistency between netcdf and grib2.