MPI based parallelism & tools, more global updates

[ffreyer]

TODO

• [x] add pmap alternative
• [x] add mpi version of ReplicaExchange
• add mpi version of ReplicaPull
• [x] figure out initialization
• [x] add manual tests?

Ref #120

I think ReplicaPull could be done with MPI windows but I'm not sure about the restrictions of it. I think you have to create a window on each process, which would be problematic with mpi_queue. I'd rather work on other things than figure this out atm