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* Performance improvments for hierarchical clustering (at the cost of memory)
* Cluster instances are now iterable. It will iterate over each element,
resulting in a flat list of items.
* New option to specify a progress callback to hierarchical clustring. This
method will be called on each iteration for hierarchical clusters. It gets
two numeric values as argument: The total count of elements, and the number
of processed elements. It gives users a way to present to progress on screen.
* The library now also has a ``__version__`` member.