Remove mpi4py as dependency
This PR removes mpi4py (again) as a dependency.
There is some history and some other issues involved. I just added a test in #100 to be able to reproduce possible failures. Before merging this PR I would like to investigate the setup described in https://github.com/precice/python-bindings/issues/8#issuecomment-575329609 again and make sure that the error is really gone (ideally also find out why it is now gone).
I don't have the time to carefully check the MPI dependency now. I'm still a bit afraid here that we might oversee something, if we merge this PR, but only having MPI as an optional dependency would still be nice. I would suggest to keep this issue. If somebody finds time to work on it or the MPI dependency really becomes a problem (because one cannot easily satisfy it on a special system, for example), we can start looking into this again.