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