feat: add Cruijff PDF

[ikrommyd]

Adds Cruijff PDF as unnormalized density
Related to zfit/zfit#512

Proposed Changes

• Add Cruijff PDF as unnormalized density

Tests added

• tests/test_pdf_cruijff.py

Checklist

• change approved
• implementation finished
• correct namespace imported
• tests added
• CHANGELOG updated

[ikrommyd]

@jonas-eschle FYI this is ready for review.

[jonas-eschle]

Looks good overall, minor things, but other than that it's fine. A pity that no analytic integral is available, but fine, the numeric integrator catches it well. I'll approve, b

[ikrommyd]

Looks good overall, minor things, but other than that it's fine. A pity that no analytic integral is available, but fine, the numeric integrator catches it well. I'll approve, b

I think there is just no anti-derivative. I tried with some integral calculators and got nowhere.

[jonas-eschle]

I've added the suggestion to refactor. I think it makes sense? If so, just apply the suggestion and the PR is approved

[ikrommyd]

@jonas-eschle there was incorrect indentation in your suggestion but I fixed it. All good now, tests pass. Will merge as soon as ci completes.

[jonas-eschle]

@jonas-eschle there was incorrect indentation in your suggestion but I fixed it. All good now, tests pass.

yep, just noticed, thanks for the fix