Building GEOSgcm with Spack

[mathomp4]

A grand hope for GEOSgcm (and for @danholdaway in particular) is one day we can do:

spack install geosgcm

and we get a build of GEOSgcm.

I'm currently trying to figure out how to get spack-stack to do that (see JCSDA/spack#174) and I've identified four paths we must look at when building GEOSgcm with Spack:

1. Build with internal MAPL and internal FMS
2. Build with internal MAPL and spack FMS
3. Build with spack MAPL and internal FMS
4. Build with spack MAPL and spack FMS

I think the FMS bits are fairly simple. I've build GEOS with FMS in Baselibs (see #368), but building GEOS with MAPL as an external library will require some...thought.

I'll probably need to think on it with @tclune at least at first. It must be doable as our UFS colleagues do find_package(MAPL REQUIRED), but do they have a FindMAPL.cmake file somewhere?