exago~mpi causes build issues

[cameronrutherford]

This is a collection of smaller individual build failures, and is capturing the higher level issue:

• exago+python~mpi fails to build #16
  • This started as a relatively basic CMake error resolve in Spack CPU build exago~mpi #47, but still won't work until ~mpi works
• exago+hiop~mpi fails to build #18
  • This might be a HiOp issue, or an ExaGO error, or both. More importantly this doesn't get caught because it isn't tested
• exago~mpi~python fails to build #39
  • Perhaps just because no-one builds this configuration, this never seems to work well