Fixed cross NN with periodic box and added tests.

[ollyfutur]

Proposed changes

Correction of a bug in utils.compute_cross_nn_distances. When using periodic conditions the function computed the self NN distances of dataset X with compute_NN_PBC instead of cross NN distances between X_new and X. This affected interpolated density estimation.

Types of changes

The cross NN distance is directly computed using cKDTree instead of calling compute_NN_PBC. I have added two tests in test_distances_utils, test_cross_nn_distances and test_cross_nn_distances_periodic.

[AldoGl]

Thank you @ollyfutur, this seems excellent work!

To me this seems immediately mergeable, but let's see what @wildromi or @imacocco say since they are the people who used periodic distances the most in the past.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d82322b) 80.29% compared to head (492fa30) 80.22%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #102 +/- ## ========================================== - Coverage 80.29% 80.22% -0.08% ========================================== Files 15 15 Lines 2431 2432 +1 ========================================== - Hits 1952 1951 -1 - Misses 479 481 +2 ```

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[imacocco]

Everything looks fine, the tests also run smoothly. It would be great also to fix the linting, then we can merge straightforwardly!! Many thanks for the correction!

[ollyfutur]

Just fixed the linting, thanks for maintaining this great package :)

[AldoGl]

Thank you @ollyfutur for entering the team of contributors! 🙂 Since tests pass and @imacocco also checked the code, I will soon rebase and merge this PR 👍🏼

[AldoGl]

@ollyfutur if it's ok for you I will add you to the list of contributors here

[ollyfutur]

Alright thank you very much @AldoGl !

[wildromi]

Thank you Olivier @ollyfutur for spotting this bug!