Looks like you're just using `LanegvinDynamicsMove` to replace the old Langevin dynamics integrator internally.

I think we want to have `ReplicaExchange` specify a default MCMC `move_set`, where the user can replace it with their own via accessing `repex_simulation.move_set = ...` if they wish.

The MCMC scheme is intended to be used by creating an `MCMCSampler` with this `move_set`, and then calling `apply()`, like this:
```python
# Create MCMC sampler
sampler = MCMCSampler(thermodynamic_state, move_set=move_set)
# Run a number of iterations of the sampler.
updated_sampler_state = sampler.run(sampler_state, niterations)
```
This will allow for the most flexibility, I think.