Use MPI-Datatypes to optimize transfer with caf_get_by_ref

[vehre]

Like in the "older" communication functions caf_get_by_ref should use an MPI-Datatype for transferring data.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.