Add MUSICA library as a build option

[mattldawson]

This PR adds a build option to include the MUSICA-Fortran library in the atmosphere model build. This is intended as a first step towards being able to use MUSICA chemistry components (MICM, TUV-x) in MPAS-A.

To include the MUSICA-Fortran library in the build, MUSICA-Fortran and its dependencies must be installed locally. Include MUSICA in the MPAS-A build like this:

make gnu CORE=atmosphere MUSICA=true

Omitting the MUSICA flag does not attempt to link against the MUSICA libraries.

I'm happy to make any changes. I did my best to follow existing patterns in the codebase, but I'm brand-new to MPAS-A.

@mgduda @rkumar-ncar @marybarthbrock @pfisterggp @K20shores @boulderdaze

[boulderdaze]

I reviewed it and it looks good to me, but I also just started learning about the MPAS way to creating makefiles by reviewing this PR, so I'd love to see comments or feedbacks from others who are more familiar with MPAS.