[WIP] TIS Equilibration & PathMover tools

Tools so we can equilibrate within the TIS ensemble, and well as a few general tools for path movers. Also gives us most of the tools for RETIS.

- [ ] `PathSampling` : calculation object for path sampling
- [ ] `PathReversalMover` (corrections and updates) : PathMover for reversals
- [x] `MixedMover` (adjustments to current version) : PathMover to select a random mover
- [x] `SequentialMover` : PathMover to run several other moves in a row (without worrying about whether previous parts were accepted). Used for, e.g., making one single move that shoots from all replicas.
- [x] `PartialAcceptanceSequentialMover` : PathMover to run several other moves in a row, until one is rejected. All accepted moves are applied.
- [ ] `ConditionalSequentialMover` : PathMover to run several other moves in a row. Only if all are accepted, are any accepted.
- [ ] Tests for all the `PathMover`s listed above
- [ ] Finish #105

(Details on various PathMovers listed will be in an upcoming issue.)