Fix python bindings for unified serial and parallel library

Created by: pnorbert

We rely on this trick to work in the type caster:

        // Import mpi4py if it does not exist.
        if (!PyMPIComm_Get)
        {
            if (import_mpi4py() < 0)
            {
                throw std::runtime_error(
                        "ERROR: mpi4py not loaded correctly\n"); /* Python 2.X */
            }
        }