[ENH] Check for distribution objects and handle them with iat/iloc

[SimonBlanke]

Reference Issues/PRs

resolves #559

What does this implement/fix? Explain your changes.

The source code changes are just the isinstance check for distribution objects and using iat/iloc for scalars or arrays. This PR also adds multiple tests and the _CompositeDistributionTester (which is used in the new tests).

Does your contribution introduce a new dependency? If yes, which one?

No

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

Any other comments?

PR checklist

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• [ ] The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.

For new estimators

• [ ] I've added the estimator to the API reference - in docs/source/api_reference/taskname.rst, follow the pattern.
• [ ] I've added one or more illustrative usage examples to the docstring, in a pydocstyle compliant Examples section.
• [ ] If the estimator relies on a soft dependency, I've set the python_dependencies tag and ensured dependency isolation, see the estimator dependencies guide.