Use MPI_THREAD_FUNNELED when available

Use MPI_THREAD_FUNNELED when available

We have never observed any problems with MPI and OpenMP,
but for compliance we should call MPI_Init_thread() and
try to get MPI_THREAD_FUNNELED support level. However,
if that level is not supported we simply call the old
MPI_Init() instead - at least for Gromacs that seems fine.
If we get an error return code we warn the user, but if
MPI_init_thread() still worked we hope for the best and
don't bother the user.

Fixes #1127.

Change-Id: I11b81a65125e32b95255dbb769cf86b835bd62ab