[WIP] Initial NCMC Implementaion

...itration object

Two modified routines have been added:  update_ncmc and setPartialTitrationState
Many more edits to come.
Actually, we don't want to use an `Integrator` at all, since we don't want to have this create a `Context` and `Integrator` when moved to the C++ layer.  You want to implement a pure Python velocity Verlet integrator inline, probably following what is done here:

https://simtk.org/websvn/wsvn/neq-mc/examples/wca-dimer/pyopenmm/run-ncmc-solvent.py

```python
            # Half-kick velocities.
            forces = state.
getForces(asNumpy=True)
            proposed_velocities += 0.5 * forces/masses * timestep 

            # Full-kick positions.
            proposed_coordinates += proposed_velocities * timestep 

            # Update force and potential energy.
            context.setPositions(proposed_coordinates)
            state = context.getState(getForces=True, getEnergy=True)
            forces = state.getForces(asNumpy=True)            

            # Half-kick velocities and update kinetic energy.
            proposed_velocities += 0.5 * forces/masses * timestep
```