Add support for MPI_Comm (mpi08) object in capgen

Description

The MPI_Comm DDT must be known to capgen to enable passing around variable of type MPI_Comm.

Summary

In trying to incorporate MPI 2008 into CAM-SIMA, @nusbaume ran into an Unknown DDT type, MPI_Comm error (coming from ~line 540 in metadata_table.py).

A few things I noticed when I was trying to understand the underlying issue:

1. Capgen does not use ccpp_types.F90 at all right now (which I believe is what prebuild uses for this purpose).
2. Capgen does not currently support the way that ccpp_types.F90/.meta grabs the MPI_Comm DDT. It expects that all DDTs for which you have metadata will be defined in the Fortran (rather than just used and made public)

Previously closed issue: https://github.com/NCAR/ccpp-framework/issues/390

Possible Solution

I'm not sure of the best solution, but a couple thoughts:

1. Build MPI_Comm into the framework as a DDT that will always be there (though perhaps we'd need a flag to turn this off in case someone is building without MPI).
2. More generally, update the parser/metadata validation to allow for DDT variables that aren't defined, but just used from an external source and made available/public (as in ccpp_types.F90 currently)

Here's the PR that enabled use of the DDT for prebuild: https://github.com/NCAR/ccpp-framework/pull/523

Start with metadata_table.py around line 540, then compare capgen’s handling with ccpp_types.F90 and .meta metadata. Review PR #523 and the previously closed issue #390 before choosing an approach. Done means capgen recognizes MPI_Comm rather than reporting an unknown DDT type.