Remove MPI from C++ code

Created by: gcarleo

At this stage, MPI is the only library we are forced to link against (except python of course), and this has proven tricky to maintain properly. Currently, MPI calls in C++ are limited to 2 main things:

• computing statistics
• seeding random number generators
• SR matrix-less multiplication

When #299 (closed) is completed, 1. won't be necessary anymore in C++ (we will have numpy-style statistics based on mpi4py). 2 can also be addressed on the python side, just slightly refactoring the C++ parallel random engine