Collections API: Implement RasterGroupedAverage

[rajadain]

Add support for the RasterGroupedAverage operation.

Sample input: nlcd-kfactor-request.json.txt

Expected output: nlcd-kfactor-response.json.txt

Depends on #51

[mmcfarland]

Include optimizations made via #47

[rajadain]

Here's an example of RasterGroupedAverage without any rasters to group by:

awc-request.json.txt

awc-response.json.txt