circumvent gurobi integer division to match cpmpy floordivision

[Wout4]

There does not seem to be floordivision in gurobi, so I implemented a workaround using continuous (gurobi) variables.

[JoD]

Interesting. So Gurobi supports integer division that rounds towards zero instead of floors? E.g., -3/2 yields -1 instead of -2? What does Gurobi yield for 3/-2? (if it yields -2 we can maybe hack that in?)

Edit: Gurobi apparently does the following decomposition: a/b becomes q with toplevel b*q=a, which indeed is a nice linear constraint if b is a constant, but it enforces a to be divisible by b.

What is the core idea of your fix? I did not immediately get it from the code - something with abs?

Would it make sense to just have a decomposition for div? E.g., a/b is decomposed to q with toplevel q*b=a+r and 0<=r<b. This will then work for Gurobi too.