add Extended Edit Distance (EED) metric

[mathemusician]

Hello all, the only thing I've not done yet is update Changelog.md. I'm assuming it should be updated after review.

What does this PR do?

Fixes #635

Before submitting

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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[stancld]

Hi @mathemusician, thanks for your contribution! :] I left some comments but we're on the way. You can also update CHANGELOG.md :]

[codecov]

Codecov Report

Merging #668 (be3814e) into master (3bd4fb0) will decrease coverage by 0%.
The diff coverage is 90%.

@@          Coverage Diff           @@
##           master   #668    +/-   ##
======================================
- Coverage      95%    95%    -0%     
======================================
  Files         167    169     +2     
  Lines        6596   6718   +122     
======================================
+ Hits         6272   6384   +112     
- Misses        324    334    +10     

[mathemusician]

Thanks for taking a look at my code @stancld! Will implement changes soon.

[mathemusician]

Thanks to everyone who reviewed my code and made this as good as it is now! :)

[Borda]

Thanks to everyone who reviewed my code and made this as good as it is now! :)

Thank you for your patience with us and for making this package even better!