Slisemap

Added a reference to the published SLIPMAP paper.

What's Changed
* Faster kmeans when plotting: https://github.com/edahelsinki/slisemap/pull/15

What's Changed

- Implement **Slipmap**, a variant based on prototypes that scales better with respect to both time and memory.
- Separate (optional) regularisation functions.
- Use Bayesian optimisation for hyperparameter tuning.
- Remove pre-1.3 deprecations.
- Add optimisation stats to metadata.
- Move paper experiments to frozen branches

**Full Changelog**: https://github.com/edahelsinki/slisemap/compare/v1.5.2...v1.6.0

Changes:

* Improved `sm.plot_dist(scatter=True)`
* Improved cluster handling in `sm.plot()`
* Added example notebook for interactive plots

Changes:

* Fix a `isinstance(bool, int) == True` oversight

Changes:
- `plot`: improve labels for matrix plot
- `plot`: more flexible clusters (not just integers [0..k])
- `plot`: bars accepts a list of variables
- `get_model_clusters`: sort the clusters labels based on the embedding
- `save` and `load`: add (optional but default) LZMA compression
- `metadata.get_variables`: fix fallback