Fix most_discriminating_terms in keyterms

[SandyRogers]

Description

keyterms.most_discriminating_terms was broken, beause of changes introduced to vsm.Vectorizer and get_doc_freqs (introduced in #167 )

Motivation and Context

I didn't make an issue for this, because it seemed like a straight forward fix.

How Has This Been Tested?

A unit test was added to test_keyterms.py, because there was previously no coverage for the most_discriminating_terms method. This was tested using pytest as standard: pytest tests/test_keyterms.py -v.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation, and I have updated it accordingly.

[bdewilde]

Thanks, @SandyRogers ! This looks great — I especially appreciate the Thomas Friedman unit test. 😂