Finally move OpenMM to contrib

I've paid lip service to attempting to leave the build functional.
There's a few places GMX_OPENMM still has to appear in the main code,
because of how CMake works and how badly GROMACS libraries work. I'm
prepared to fix obvious major issues anyone spots. I'm proposing not
to do any testing of whether the build works, since the 4.5 build is
still available. If later on someone wants both OpenMM and some
feature in 4.6 they can ask then for help fixing the damage here.

Fixes #1079, for certain values of "fixes" :-)

Change-Id: Ic27b8c3bdc1255cc3edb01bcfa11f47d6a03b44d