Generalize Jin Xin relaxation

[ranocha]

I drafted a generalized version of Jin Xin relaxation that should also work for other equations - at least the stuff in the equations directory, not the implicit stuff. How does that look to you, @sloede?

[github-actions[bot]]

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[sloede]

I decided to merge this as it is now but for now I reverted the behavior to what it was before.