In Naive Bayes, avoid using `Option::unwrap` and so avoid panicking from NaN values

[corebreaker]

Fixes #273

Checklist

• [X] My branch is up-to-date with development branch.
• [X] Everything works and tested on latest stable Rust.
• [X] Coverage and Linting have been applied

Current behaviour

Panics in function naive_bayes::BaseNaiveBayes::predict

New expected behaviour

No panics with NaNs

[codecov-commenter]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (9c07925) 45.04% compared to head (00fb486) 44.95%.

Files	Patch %	Lines
src/naive_bayes/mod.rs	35.71%	9 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #274 +/- ## =============================================== - Coverage 45.04% 44.95% -0.10% =============================================== Files 85 85 Lines 7206 7214 +8 =============================================== - Hits 3246 3243 -3 - Misses 3960 3971 +11 ```

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[morenol]

Thanks @corebreaker, can you add an unit test for that?

[corebreaker]

Sure, @morenol, i'll add an unit test.

[corebreaker]

@morenol , unit tests were added.

[corebreaker]

Sorry i forgot, review process.