Add clustering algorithms

[josevalim]

• K-means
• Affinity Propagation
• Mean Shift
• Spectral clustering
• Hierarchical clustering
• DBSCAN
• HDBSCAN
• OPTICS
• BIRCH

[grevgeny]

I'm interested in contributing to the project and would like to begin by implementing the Mean Shift algorithm.

[josevalim]

Please go ahead @grevgeny! You can see the PRs from other clustering algorithms, to get an idea how of these contributions usually work (and the project goals).

[paulsullivanjr]

I'm going to work on implementing BIRCH