Add 'supported' CAM-MPAS 120km 32z uniform mesh compset for CESM 2.3 release

This issue is to help coordinate adding 'support' to a 120 km 32 vertical levels MPAS mesh. The 120 km mesh has 40,962 grid columns.

To Do:
* Add MPAS Grids to CIME Head
* Add MPAS Grids to CTSM Head
* Add MPAS Grids to SVN Repo
* MPAS 120km 32z with no topography (?)
* MPAS 120km 32z with topography
* MPAS 120km 32z with topography and initial conditions
* January 1st, 2000 CSFR initial conditions? Or spun up from `us_standard_atmosphere`
* MPAS Decomposition Files (See Below)

**MPAS Decomposition Files**

Stand alone MPAS bundles the following decomposition files with the 120 km mesh: 2, 4, 6, 8, 12, 16, 25, 36, 48, 64, 96, 128. We generally try to bundle task counts at which MPAS runs reliably well at (300-600) with a few at the lower end for users on personal laptops.

However, F-Compsets may not run very well with lower tasks counts (2-16 maybe?) on the 120km. F2000 climo compsets with CAM-MPAS are generally run on cells per tasks counts around 20 - 300. I'll produce a list of decomposition in a later comment as a first go.