Return grid data by arbitrary polygon

[njmattes]

Looks like we have methods for returning data either by the uid of a stored region, or we're returning all the data for the globe. (This latter request seems to time out before it completes.) We also need a way to retrieve a selection within an arbitrary rectangle.

[Edit 28 May 2016: Per our last Monday meeting, even when retrieving data for the viewport (rather than a user-specified shape) this should probably be an arbitrary polygon of any sort, rather than merely a rectangle.]