Balanced kmeans (WIP)

[stuartlynn]

• [ ] All declared geometries are geometry(Geometry, 4326) for general geoms, or geometry(Point, 4326)
• [ ] Existing functions in crankshaft python library called from the extension are kept at least from version N to version N+1 (to avoid breakage during upgrades).
• [ ] Docs for public-facing functions are written
• [ ] New functions follow the naming conventions: CDB_NameOfFunction. Where internal functions begin with an underscore
• [ ] Video explaining the analysis and showing examples
• [ ] Analysis Documentation written template
• [ ] Smoke test written
• [ ] Hand-off document for camshaft node written
• [ ] If function is in Python, code conforms to PEP8 Style Guide

[andy-esch]

A potential problem I see coming for this is that it's tied to scikit-learn 0.17.x, not 0.17.x+. Current version of scikit-learn is 0.19.x, which we'll probably want to include when we update the Python libraries..