Implement BayesianMarkovStateModel

Clear PR with squashed history for `BayesianMarkovStateModel`.

- [x] Implement sampler
- [x] set `BayesianMarkovStateModel` API
- [x] Add docs
- [x] Add example
Could be useful, as that would allow us to make sure that all our normalizations are consistent between different models.  You could also just file and issue and make someone else do this in a separate PR too.