Add (optional) topology to testSystems about openmmtools HOT 4 OPEN

See #8

Also, we should probably not impose an MDTraj requirement here, so it will have to be an openmm topology for now. It's easy enough to convert to MDTraj from OpenMM...

from openmmtools.

Actually, I suppose we could also expose an MDTraj topology if we keep use local imports to avoid a install-time dependency.

from openmmtools.

openmm.Topology is fine. mdtraj can convert easily enough. If there is no objection I would add the topology to some of the examples that I use. We can see if that makes sense.

from openmmtools.

Let's use the OpenMM Topology for this.

from openmmtools.