Add basic CI using GitHub Actions

[mwarusz]

This PR adds basic CI for PAM using GitHub Actions.

The CI currently does three checks:

• builds and runs PAM-C unit tests.
• builds (but not runs) PAM-C idealized solvers for different combinations of models, hamiltonians, and thermodynamics.
• builds and runs PAM-C and PAM-A mmf_simplified driver for two GCM time steps with YAKL_DEBUG turned on.

All of this is tested on Ubuntu with both GCC and Clang and on MacOS with Clang, except for PAM-C compilation tests
which are done only on Ubuntu using GCC to avoid creating too many jobs.

This is very bare-bones, but I think it is a decent start. Notably, there are no tests with CXX physics. Suggestions for
more and better tests are very welcome.