When creating composites, allow loading and saving to YAML

[gerritholl]

In SIFT we can either create composites from scratch, or load them as we load channels. It would be nice if:

• In the composite creation dialogue, we could pre-load the definition of a satpy-defined composite, and
• we can store the composite defined in the result as a Satpy composite (+ enhancement).

This would only work for a subset of composites, perhaps those using "simple" RGB compositors. It might also require that the SIFT Composite creation dialogue can work more generally with enhancements and modifiers; so far it seems to support only a gamma correction and no modifiers.

[djhoese]

Very interesting idea. It might be hard to get it right and having to tell users they could only do "simple" RGBs for now might be difficult. I like the idea and it would be useful, but this may require utilities in Satpy to update YAML configurations at runtime. Sort of as a convenience for Satpy users that SIFT just happens to take advantage of.