Feature/comm tests (#201)

Add unit tests for communicator

fixes #200

    update global_communication test driver to initialize correctly in
    dry run mode
    add unit tests that test the communication::global_policy:
        basic initialization
        global spike exchange (just the spike gather step, not the
        event delivery).
    improve the formatting of the reporting from the MPI GTest wrapper
    to make it easy to see if tests have failed.