Require explicit MPI_COMM for gmx_bcast and gmx_barrier

Require explicit MPI_COMM for gmx_bcast and gmx_barrier

This changes gmx_bcast and gmx_barrier to take the MPI communicator
explicitly instead of taking a pointer to t_commrec and using
mpi_comm_mygroup. This also allows to remove gmx_bcast_sim and leave
the responsibility of passing the right communicator to the caller.

This is a first step in breaking up t_commrec. These functions
are the subset of low-level networking functions which are used
before domain decomposition (and hence PP/PME ranks) is set up.

Refs #2395