Enable separate PME GPU rank

Enable separate PME GPU rank

This relaxes a few task assignment restrictions to allow
a simulation with a single separate PME GPU rank to work. E.g.

gmx mdrun -pme gpu -ntpmi 4 -npme 1

works if all the single rank PME GPU requirements are satisfied.

Default behavior is not changed, new behavior is opt-in.

The existing single-rank PME command line sanity test is altered
to also be included in multi-rank tests, testing a separate PME rank.

Change-Id: I82f3950b0e04b0bc21843a8124a9bd5c68b15024