MPAS applications: fix / add several aviation fields, remove "if imp_physics..." logic from INITPOST_MPAS

[jaymes-kenyon]

A forthcoming PR will do the following for MPAS-related applications:

• For VIL, discontinue use of the local GRIB2 table
• Add GSL's experimental ceiling fields to the GRIB2 output
• Within "INITPOST_MPAS", remove instances of "if imp_physics...". These portions of the code initialize various arrays based upon the microphysics scheme. However, many of these "imp_physics" identifiers correspond to schemes in WRF-ARW. If they are again needed for MPAS, these codes can be re-introduced.

This issue can be assigned to @jaymes-kenyon