creates new moran's i functions which expose spatial lag

[andy-esch]

This PR adds new functions which expose the spatial lag for Moran's I local (regular and rate). We need this functionality to create data science dashboards that communicate the moran's i scatter plot (described here). This also starts the process of deprecating the existing Moran's I functions (see #196 for more) in favor of ones with better names:

• CDB_AreasOfInterestLocal --> CDB_MoransILocal
• CDB_AreasOfInterestLocalRate --> CDB_MoransILocalRate

Note this changes the internal function _CDB_AreasOfInterestLocal (and ...Rate) to ~return the spatial_lag column~ to rely on the _CDB_MoransILocal function instead. The existing public functions CDB_AreasOfInterestLocal and CDB_AreasOfInterestLocalRate are not affected (signatures remain the same), but their internals were refactored to accommodate the addition of the CDB_MoransILocal and CDB_MoransILocalRate functions.

ToDo

• [x] awaiting info about whether to return standardized value (and standardized spatial lag) -- decision to included both

Other ToDos

• [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] New functions follow the naming conventions: CDB_NameOfFunction. Where internal functions begin with an underscore
• [x] If function is in Python, code conforms to PEP8 Style Guide

edit removed some of the PR items that are now irrelevant

closes https://github.com/CartoDB/crankshaft/issues/201

[stuartlynn]

This looks good to me. Think we are good to merge this in