Add TER

[stancld]

What does this PR do?

This PR adds a new text evaluation metric - TER.

Fixes #636

I write _LevenshteinEditDistance in helper.py as I think it can be also utilised for EED.

Current state: I need to go through the PR tomorrow once again and then it should be ready for review.

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?

PR review

Anyone in the community is free to review the PR once the tests have passed.
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[codecov]

Codecov Report

Merging #646 (c26e243) into master (965a6cf) will increase coverage by 22%.
The diff coverage is 94%.

@@           Coverage Diff           @@
##           master   #646     +/-   ##
=======================================
+ Coverage      72%    95%    +22%     
=======================================
  Files         164    166      +2     
  Lines        6002   6369    +367     
=======================================
+ Hits         4350   6045   +1695     
+ Misses       1652    324   -1328