Change indexing in UnstructuredRepertoire intra batch comp

[LisaCoiffard]

Related issues: N/A

Quick fix to select the current fitness in intra batch comp in the add method of unstructured repertoire. The current indexing will fail if any of the descriptors in the batch have nan values (e.g. when re-initialising the repertoire in AURORA, after training the auto-encoder).

This PR introduces:

• change indexing of fitnesses to use current_index in intra batch comp rather than 0th knn_relevant_indices.

Checks

• [x] a clear description of the PR has been added
• [ ] sufficient tests have been written
• [ ] relevant section added to the documentation
• [ ] example notebook added to the repo
• [ ] clean docstrings and comments have been written
• [ ] if any issue/observation has been discovered, a new issue has been opened

Future improvements

[List here potential observations made and/or improvements that could be made in the future. If relevant, open issues for those.]

[Lookatator]

Hi @LisaCoiffard, thank you very much for the PR! Could you fix the style issue in the CI, so that I can merge it? :D