Looking at removing the requirement for mpi4py (making it optional), following user discussions.

For:
Installing mpi4py can be problematic and is often not needed.

Against:
An argument for keeping it might be that even if you don't use it, many of the examples including most of the example simulators (every case that uses the template evaluator) uses libEnsemble’s MPI executor and therefore requires an MPI. So we could make MPI a required dependency, but then might it be just as simple to keep mpi4py required?

I have not updated the platform Specific Instructions