allow for mpi4py version 2 or 3 in Python wrapper

  def __str__(self):

    return repr(self.message)

class lammps(object):

  # detect if Python is using version of mpi4py that can pass a communicator

  has_mpi4py_v2 = False

  has_mpi4py = False

  try:

    from mpi4py import MPI

    from mpi4py import __version__ as mpi4py_version

    if mpi4py_version.split('.')[0] == '2':

      has_mpi4py_v2 = True

    if mpi4py_version.split('.')[0] in ['2','3']: has_mpi4py = True

  except:

    pass

      # need to adjust for type of MPI communicator object

      # allow for int (like MPICH) or void* (like OpenMPI)

      if lammps.has_mpi4py_v2 and comm != None:

      if comm:

	if not lammps.has_mpi4py:

          raise Exception('Python mpi4py version is not 2 or 3')

        if lammps.MPI._sizeof(lammps.MPI.Comm) == sizeof(c_int):

          MPI_Comm = c_int

        else: