hotfix - enable CMake to checkout frozen references for physics_mmm and UGWP

[guoqing-noaa]

As discussed in PR #230 , CMake always check out the latest branch of physics_mmm and UGWP.
Apparently some recent changes in physics_mmm break the CMake build system as reported in #228

The expected good solution will be that we use git submodule universally to manage all MPAS-Model submodules and retire all other nonstandard methods. But it is uncertain when this will happen.

Before that, this PR provides a hotfix on src/core_atmosphere/CMakeLists.txt to enable it to checkout frozen references for physics_mmm and UGWP so that we can compile MPAS-Model successfully using CMake.

Mandatory Questions

• Does this PR include any additions or changes to external inputs (e.g., microphysics lookup tables, static data for gravity-wave drag -- things like that)?
  • no
• Does this PR require updating one or more baselines for the CI tests? If so, what?
  • no

Priority Reviewers

• @clark-evans
• @dustinswales

[dustinswales]

Replaced by #233