558 reporting intermediate solutions

[IgnaceBleukx]

Following the discussion in #558.

It adds quite some code in all solver interfaces, but I tried to do it somewhat consistent.
Some solvers are missing still:

• minizinc - this needs a lot of engineering with async functions (similar to solveAll)
• gcs - there is a callback function but as optimization is not working properly (see Objective in GCS does not have any influence on solution quality #559) I could not test it.

For Exact,it is somewhat hacky as we need to go through "runOnce" instead of the nice "toOptimal" function. So there will be some more ping-pong through the interface.

[ThomSerg]

Seems a nice quality-of-life improvement. But yeah, the one for exact seems hack-y, especially with the "warning" part. Is there a way to restore the solver afterwards?

[ThomSerg]

Besides that it is never mentioned + silently enforced that "display" is only for optimization, it seems good to me.

[Wout4]

we could add the gcs callback also since that bug is fixed or not?

[IgnaceBleukx]

Made quite some changes to this PR after the last review, so maybe another round of revieuwing from @ThomSerg and/or @Wout4 would be appropriate before merging.

I'm wondering if we should really fill the solver.objective_ argument directly from the underlying solver object.
Alternatively, we can store the original objective expression and compute the objective value based on that. There were quite some tricky things to get right in this PR...

[IgnaceBleukx]

I think the comments are addressed @ThomSerg

[IgnaceBleukx]

I decided to take out the display support for Z3 after all.
It was too unreliable to be tested properly (z3 finds a first solution relatively quick, but then does not improve, even with a large time out).

Alternative is to keep it supported, but skip z3 in the tests?
In any case ready for a round of reviewing.

[ThomSerg]

Very minor styling comments. For the rest looks good!

[IgnaceBleukx]

Processed comments and merged master into this one; but its missing a PR "approval" :)

[ThomSerg]

Approved