Support for constraints in RandomVelocities

[dwhswenson]

Previously, the RandomVelocities snapshot modifier did not account for constraints. This could cause problems in committor simulations (or aimless shooting, when we add that).

This PR adds OpenMMEngine.apply_constraints. This does not require that other engines (e.g., Toy) include an apply_constraints method. That keeps the simplest engines simple, in cases where there's no such thing as constraints.

• Add apply_constraints function to OpenMM engine
• Use snapshot.engine.apply_constraints(snapshot) in RandomVelocities, within try: except:
• Tests
• Update docs/docstrings

[dwhswenson]

Tests pass; docstrings done. Ready for review.

[jhprinz]

very nice. Will merge.