Make a land-sea compositor about satpy

[djhoese]

Sometimes readers could provide the land-sea mask (some level 3 files have them). This could be treated similar to how @pnuu wanted solar zenith angles to work. Meaning they could be automatically generated if they weren't available in the reader.

Also maybe we could consider this a "where" compositor like np.where(condition, a, b).

[pnuu]

The "where" compositor sounds like a good idea. It could be used to threshold IR imagery with cloud mask or a threshold and replace those areas with a static background image. This is something that we've had for our public pages [link]. Although I personally don't like them, I still might need to create them...

So, to complement the proposal @mraspaud made, it'd be nice to have a possibility to use external files with the mask, NWC SAF cloud mask/cloud optical thickness or similar other products to create the maskin.

[pnuu]

Hmm, I think the MaskingCompositor in #1175 actually added the support needed to implement this. Just use a land-water-mask for the conditions and stack few composites to get the desired result.