Add ability to run UA systems

We can get a pretty big performance bump if we try out some of these advanced sampling techniques using united atom (UA) systems instead of all-atom (AA).

There are methods in both polybinder and placnkton that can remove the hydrogens. A couple things to keep in mind:

1. The heavy atom the hydrogens are bonded to should absorb the hydrogen's mass and charge (if we're using charges)
2. We have to keep the hydrogens in place when applying a forcefield, so removing them needs to be done in the Parmed structure.

We can probably have a class method in `system.System` that does this.