Python cleanup
Created by: germasch
Since I'm at it, let's dump on more PR here. This one is on top of #1247, so shouldn't be considered before that one has been worked out.
This PR adds a pybind11 caster to make passing MPI4PY communicators much simpler. It also converts
ADIOS
back into an actual Python object, rather than a fake-constructor that returns a py11_ADIOS
object.