Added a utility method to the Learner class returning feature names

[remo-help]

This PR adds a simple utility method to the Learner class, allowing it to return the feature names used at the estimator step as a 1-D array. Learners with a selector potentially have a smaller feature name list than is stored in the feature_vectorizer. This method allows for easy access of the actual feature names of features used, even if the selector has removed specific features.

[remo-help]

hold off on review while I add tests

[codecov[bot]]

Codecov Report

Base: 96.90% // Head: 96.90% // No change to project coverage :thumbsup:

Coverage data is based on head (bdf6107) compared to base (bdf6107). Patch has no changes to coverable lines.

:exclamation: Current head bdf6107 differs from pull request most recent head f3d5658. Consider uploading reports for the commit f3d5658 to get more accurate results

Additional details and impacted files

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  Coverage   96.90%   96.90%           
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  Files          63       63           
  Lines        9271     9271           
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  Hits         8984     8984           
  Misses        287      287           

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