Colormap support 

This adds backend support and UI elements for selectable colormaps. This currently only works for the NDVI mode, but should probably also be used for the NIR view.

Also, @jywarren, this raises the question of where should the canonical repository (the code running on the server) for this project reside? I think it would make sense if it were the PVOS repository. This would mean that pull requests against the master branch of the PVOS repository would be requests to bring code live. Alternatively, we could maintain a `live` branch in the PVOS repo. Either way works for me.