validate map strategies for MPI maps

[mmckerns]

Migrated from: http://trac.mystic.cacr.caltech.edu/project/pathos/ticket/89

[mmckerns]

Updated the link: https://uqfoundation.github.io/project/pathos/query.html (none of the ticket links work, however). The point was that the MPI map strategies ("scatter-gather" versus "worker pool") should be checked for bugs, and potentially have tests specifically validating that they work as expected.