Add support for MPI.IN_PLACE to Ensemble methods.

[JHopeCollins]

Issue:
The Ensemble methods always assume that they are being passed a Function, even when the MPI function they are wrapping will accept MPI.IN_PLACE as one of the arguments (e.g. reduce). It would be useful to be able to do these operations in place where MPI supports it.

Solution:
The Ensemble would test the arguments to the appropriate methods to see if it's a Function or MPI.IN_PLACE, and handle each appropriately.

Alternatives:
Users reach in and use the Ensemble.ensemble_comm methods themselves when they want to use IN_PLACE, which would be error prone and annoying.