add controls for band selection, etc

[cholmes]

For COG's that are more than three bands it would be good to have a little GUI element to let users select the RGB bands. Planet visual by default in particular uses the first band for a UDM, that messes things up.

You can just add a &rgb= to the url string, but that's not easy to know. It'd be nice to have a little 'advanced' dialog that lets you select the bands you want to visualize. Would be cool if it could eventually let you do other tweaks, like do ndvi and pick histogram cuts, like on remotepixel.ca