Add is_manager attribute

[shuds13]

Add support for manager only operations when running with MPI communications.

Note that when running with MPI comms, user calling scripts will need to run post-processing operations using this e.g.,

    exploration.run()

    # Get generated points.
    if exploration.is_manager:
        h = exploration.history
        h = h[h["sim_ended"]]
        x0_gen = h["x0"]
        x1_gen = h["x1"]

• [x] Add tests using MPI comms

Supports #203

[shuds13]

I think I've addressed the comments. Feel free to modify my wording.

[AngelFP]

Any idea why the openMPI tests are failing? I don't see why this happens by looking at the changes. Could it be related to pytest-mpi?

[shuds13]

Yes the failure was caused by my adding of an mpi4py import in the new test. This gets imported by pytest at the global level (for all tests) and created a nested MPI environment, causing openMPI tests to fail. To do this, test would need to store mpi_comm in exploration.

[AngelFP]

Thanks for checking that! The problem I see now is that the new test will pass no matter what the value of is_manager is, right? Is there any way we could check that it is actually working as expected?

[shuds13]

I've confirmed without the if exploration.is_manager: the test fails with IndexError, so this test verifies manager only behaviour.

[AngelFP]

Great, thanks for checking. Then we can merge this as is.