Add nonspatial kmeans w class framework

[andy-esch]

• [x] All declared geometries are geometry(Geometry, 4326) for general geoms, or geometry(Point, 4326)
• [x] 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).
• [x] Docs for public-facing functions are written
  • [x] SQL
  • [x] Python
• [x] New functions follow the naming conventions: CDB_NameOfFunction. Where internal functions begin with an underscore
• Smoke test written -- will handle in a separate issue
• Hand-off document for camshaft node written -- will handle in a separate issue
• [x] If function is in Python, code conforms to PEP8 Style Guide
• [x] Appropriately mark function's parallel safety

[andy-esch]

@mehak-sachdeva, could you give this a code review, please?

[andy-esch]

@ethervoid, this one is ready for queuing up for a deploy. could you give it a look over please?

This will be one of three or four PRs this week that will be ready for a deploy.

[andy-esch]

@ethervoid, any interest in still reviewing this? It's pretty similar to existing plpython analysis functions, so no curve balls here.

[andy-esch]

Thanks @ethervoid :)