Aligned array

- Fix bug in `PitmanYor::CachedMixture` and add regression test
- Add `ArrayFloat` to allow passing numpy.ndarray through to lp scoring functions
- Add `.packed_add()` and `.packed_remove(id)` methods to `VectorFloat` to simplify mixture logic
- Add `DIST_ASSERT_ALIGNED` and similar macros
- Add generic `MixtureSlave` class