Add algorithms selection to /cog/viewer and custom expression support

[jo-chemla]

Following #1116 extract /cog/viewer PR into this one

Below comment now tackled!

e.g algorithm might need more than one band, so I don't think they should be in the 1b viz

Note from Vincent: FYI I've started a PR in rio-viz to support raster-dem viz developmentseed/rio-viz#64 ;-)

[jo-chemla]

Quick note, could not find where icons are taken from, tried functions/calculator/etc but no luck.

[vincentsarago]

I think they are from https://labs.mapbox.com/assembly/icons/

[jo-chemla]

This one should be good to merge as well from my point of view, don't hesitate if you have any feedback or would like to remove some things (like expression support)!

[vincentsarago]

as I said earlier I'm trying to find spare time to do Open source work but for now I have some paid project work to do I'm sorry @jo-chemla

[jo-chemla]

Hi again, sorry for the bad wording, just wanted to add that final comment for validation after all my commits, and of course no need to rush, this PR should follow the standard merge/release cycles, whenever time allows. And thanks for all the great work as always!