Caller Pass MPI_Comm

[AlexBuccheri]

Allow a caller to initialise the MPI environment, and pass a communicator to Fortuno

@aradi:

1. How would you like to control whether Fortuno finalises? Have an internal attribute in type :: mpi_env that logs where the comm came from?

2. I assume I should also add a test?

[hmenke]

Should final_mpi_env call MPI_Finalize even if the communicator was user-provided?

[aradi]

I think, this approach is more involved, than I thought originally. If the user sets up the MPI framework before calling Fortuno, Fortuno should not finalize it, but return. This includes some complications:

• User should call run_mpi_cmd_app instead of execute_mpi_cmd_app, as latter exits before returning (and ensures, that the right exit code is passed to the OS).
• Apart of tearing down the MPI, the caller also has to make sure, that the exit code returned by run_mpi_cmd_app is passed to the OS, otherwise Fortuno might exit with the wrong exit code, fooling the test system.

Therefore, I merged #50, which gives an easy way of threading level selection. I think, it would be still worth to add the feature of fully user controlled MPI-framework creation/destruction, but via then via the run_mpi_cmd_app function.