Presolve rules and documentation template

[mtanneau]

WIP. Early feedback is welcome.

Once this is satisfactory, I'll move to the other rules.

[joehuchette]

Looks very good to me, just some minor comments on the comments.

[mtanneau]

Last call for objections, otherwise I'll merge this tomorrow morning.

[mtanneau]

I just realized the infeasibility check can be tightened for binary variables (the variable should be fixed to either 0 or 1, not just an integer).

I'll add this before merging.

[mtanneau]

I think I'm 99% satisfied now 😅
I made the docs for FixSingleVariable as extensive as possible, so it can be used as template for other rules.

The last 1% covers whether parts of that doc should rather be tied to the FixedVariable reduction, but I'll leave that for future work.

[joehuchette]

Looks great!