This is based on our discussion in #889 .

It adds:

• parsing of gromacs mdp files (including types for most/all options we would want to use/modify from ops)
• snapshot_timstep property (in ps) as calculated from the parsed mdp
• write_mdp method for gromacs engine to write (modified) mdp files
• apply_restraints method for gromacs engine
• randomize_velocities method for gromacs engine (I would add this here since it feels natural to me to do it in one go with the write_mdp and apply_restraints)

Some things to note:

• I went with lists as values for every gromacs mdp option. This is due to the fact that gromacs allows multiple values for most mdp options. I did not treat the options which only have one value ('dt', 'nstxout', etc.) specially, this will also make the write_mdp method much simpler.