Add concave hull and convex hull as SoQL functions

Background for this is this issue raised from the customer summit first look dataset prep:
https://socrata.atlassian.net/browse/CORE-3607

Basically, we need a smarter way to pull the subset of georegions to display on a choropleth. Obviously, these two functions are going to be more expensive than simply using the extent function, especially on really large georegion datasets; I'm not sure how expensive, but I'm implementing them in order to do some perf comparisons so we can understand the tradeoff.