Update similarity module

[bdewilde]

Description

• add character ngram-based similarity measure, for something that's useful in different contexts than the other measures
• remove jaro-winkler string similarity measure, since it was just a wrapper around the underlying implementation and didn't add much beyond other measures
• make token sort ratio faster
• replace python-levenshtein dependency with jellyfish, for its active development, better documentation, and compliant license
• improve tests and tidy up docs

Motivation and Context

The incompatible license issue has been an outstanding to-do for a long time. Otherwise, just the usual house-cleaning. :)

How Has This Been Tested?

Refactored existing tests, wrote a few new ones, and all are passing.

Types of changes

• 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:

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