Add defaults for various MPAS-A meshes in namelist_defaults_cam.xml as unsupported configurations

At present the MPAS-A dycore in CAM can be run with only a patchwork of various resolutions (meshes) and compsets. Moving forward, it would be desirable to be able to configure the MPAS-A dycore for the Cartesian product of resolutions `{mpasa480, mpasa240, mpasa120, mpasa60, mpasa30, mpasa15, mpasa15-3-conus, mpasa7.5, mpasa3.75}` and compsets `{FHS94, FKESSLER, QPC6, F2000climo}`. These configurations do not need to be *Supported* at present.

- Regarding the production of bnd_topo files:
- For climate configurations / "CAM workhorse" configurations, it's not clear yet what smoothing radius should be used for topography
- For producing analyses using DA, NWP time-scale applications, etc., we tentatively plan to use bnd_topo files produced using half the smoothing radius that is typically used for other dycores; e.g., produce topography for the 7.5-km mesh with an `nc6000` intermediate cubed-sphere grid and a smoothing radius of `Co0004`.