capgen: Add capability and test for passing mpi communicator as a flat field

[climbfuji]

Description

SIMA folks reported that they can't pass the MPI communicator as a variable with metadata when it is of type mpi_comm, because capgen insists on a definition of the DDT in a module and metadata file.

Solution

We need to make sure that capgen can pass such an MPI communicator and correctly add

use mpi_f08, only : mpi_comm

to the auto-generated code where needed.

We should also add a simple test for this to the CI testing harness.

Alternatives (optional)

No alternative. Can't stay with or go back to old integer MPI communicators.

Related to (optional)

n/a

[climbfuji]

@nusbaume @peverwhee @dustinswales I came up with this syntax in the CCPP metadata that handles the MPI communicator and any other external DDT fine. I think this is easy to implement:

[ mpi_comm ]
  standard_name = mpi_comm_whatever_the_standard_name_is
  long_name = ...
  type = external:mpi_f08:mpi_comm

The key is type = external:mpi_f08:mpi_comm, or type = external:<module_name>:<type_name>.

When the parser finds that, it can add the DDT to the library of known DDTs together with the information on where to import it from.