Add support for custom MPI communicator

The python bindings currently don't support the constructor which provides a custom MPI_Communicator to preCICE.

https://precice.org/doxygen/develop/classprecice_1_1Participant.html#a292d1a7e046ac3c083c3eec09bdb1a48

This locks preCICE into using MPI_COMM_WORLD and blocks reconstruction of the Participant.

Resulting issue: https://github.com/precice/precice/issues/828