Duplicate compilation of shared MPAS infrastructure

[gdicker1]

In EarthWorks compsets with multiple MPAS cores ("FullyCoupled" or any case with MPAS-A dycore plus MPAS-SI prescribed mode), each MPAS core compiles its own copy of the shared MPAS infrastructure and links against that. This leads to inefficient use of build-time and lead to "Multiple defintion" errors during the final linking step (that were seen in #36).

This issue serves as reminder (mainly to me) to fix this in the build so that only one copy of the shared MPAS infrastructure is built per case.

(NOTE: "shared MPAS infrastructure" refers to the code that is not core-specific in MPAS. Essentially the contents of the src directory in the EarthWorksOrg/mpas-framework repo.)