Issue/classifier classes/test

[m5signorini]

Tests for securing the existence of the classes_ attribute in a classifier after calling the fit method.

[m5signorini]

I have made the corresponding changes, but I am not sure if the typing in the list of tested classifiers is correct. I assume using ClassifierMixin[FData, NDArrayAny] is fine.

[codecov[bot]]

Codecov Report

Base: 85.28% // Head: 85.31% // Increases project coverage by +0.02% :tada:

Coverage data is based on head (2ff203d) compared to base (5c3c139). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #481 +/- ## =========================================== + Coverage 85.28% 85.31% +0.02% =========================================== Files 137 138 +1 Lines 11044 11065 +21 =========================================== + Hits 9419 9440 +21 Misses 1625 1625 ``` | [Impacted Files](https://codecov.io/gh/GAA-UAM/scikit-fda/pull/481?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM) | Coverage Δ | | |---|---|---| | [skfda/tests/test\_classifier\_classes.py](https://codecov.io/gh/GAA-UAM/scikit-fda/pull/481/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM#diff-c2tmZGEvdGVzdHMvdGVzdF9jbGFzc2lmaWVyX2NsYXNzZXMucHk=) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GAA-UAM)

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