Clean up multisim, communication, and REMD code

Clean up multisim, communication, and REMD code

Moved multisim declarations and code to one place.

Moved mpi_in_place_buf_t to its own files in a low-level module, and
made sure we tested a build that needed it. Removed duplicate tng
option declaration while I was there.

Moved a check for REMD to its code, removed another that was redundant
with that for multisim requiring more than one simulation.

Added some t_commrec and gmx_multisim_t cleanup code, but not all
of it can work before some more cleanup takes place.

Change-Id: Ie591f50a2162dce2e83552102f8633c8d49553a7