functions should be agnostic to 'id' and `the_geom` where appropriate

[andy-esch]

The original k-means function (

crankshaft/src/py/crankshaft/crankshaft/clustering/kmeans.py

Lines 20 to 21 in b547d35

	"geom_col": "the_geom",
	"id_col": "cartodb_id"}

) does not take an optional database id -- it must be cartodb_id. This should be flexible so the extension can be used on databases outside of carto more easily.