[WIP] OptionalEnsemble and SlicedTrajectoryEnsemble

This will resolve #80 and resolve #82.

Task list:
- [x] Name change: `AlteredTrajectoryEnsemble` => `AlteredEnsemble`
- [x] Implement `OptionalEnsemble`
- [ ] Tests for `OptionalEnsemble`
- [x] Implement `SlicedTrajectoryEnsemble`
- [x] Tests for `SlicedTrajectoryEnsemble`
- [ ] `SlicedTrajectoryEnsemble.__str__` and test for it
- [ ] Example ipynb for combining `SlicedTrajectoryEnsemble` with `SequentialEnsemble`