Variations of stages for systematics

[kreczko]

Let's talk systematics: https://fasthep-flow.readthedocs.io/en/latest/configuration/variations.html

This was the last undocumented piece for the full release, everything else will have to come later.
As usual, interested if that's approximately what people want, happy to modify (or you can) if needed.
And, of course, this is all vaporware at the moment. I will start implementing the lower level things from now on.