v0.2.0

New:

• Added the Clustering API. Safely perform k-means clustering and retrieve the centroids of each cluster.
• Added a more type-safe FlatIndex API, which enables fetching the indexed data and calculating distances on subsets of the index.

Breaking changes:

• The main error type Error is now an enum rather than an alias to NativeError.
• Fetching the pointer to the underlying implementation is always done through inner_ptr(), inner_ptr_mut() no longer exists.

Other:

• Improved implementation of GPU index: going back and forth between CPU and GPU retains static typing.
• Improved documentation.