Create gromacs/mdrun module and move code there

Create gromacs/mdrun module and move code there

Improving how we dispatch the work for do_md and friends requires that
they all compile as part of libgromacs (or all outside of it). The
code currently in src/programs/mdrun is there simply because it hasn't
been moved from the old src/kernel layout, rather than through any
clear design, so we should move it now that we have a reason.

Other code from src/programs/mdrun now needs to move into libgromacs
also, but we currently don't have a module for miscellanous mdrun
features, so we're adding to the abuse of mdlib for now.

Refs #1793
Refs #2423

Change-Id: I4f9ad5d0bf6585675472b49b2d5654f588b7214e