Enable cardinality constraint in botorch recommender

[Waschenbacher]

This PR enables that cardinality constraints are supported in botorch recommender. It is on the top of #270

• ContinuousCardinalityConstraint has a to_botorch method implemented. The continuous cardinality constraints are relaxed to nonlinear inequality constraints, which is supported by botorch's optimize_acqf.
• The main idea of the relaxation: we use a narrow gaussian to check whether a value is zero or not.

[Waschenbacher]

Hi @AVHopp @AdrianSosic @Scienfitz, according to the CI failure I have to fix sth in this PR, so please hold off on the review. I will let you know when it is ready.

[Waschenbacher]

Hi @AVHopp @Scienfitz , this PR is currently on hold. I had a call with @AdrianSosic and we would need some time to fix. Will let you know when it is ready for review.

[AVHopp]

@Waschenbacher @AdrianSosic would you be fine with turning this into a draft PR until it is ready for review?

[AdrianSosic]

@Waschenbacher @Scienfitz @AVHopp: I'm closing the PR now because we there are other things regarding cardinality constraints we should tackle first, i.e.:

• finishing #394
• adding an example
• adding a user guide

While working on the above, we can think about how to continue working on other optimization approaches. However, I propose to have a dedicated baybathon with our guest @Waschenbacher in the Hufhaus first so that we can brainstorm ideas and pros/cons. Until then, I'll close here. We can always reopen if needed