Improvements to byte pair encoding vectorizer

[hamelin]

• More tests
• Setting of max_char_code as a parameter
• Parallel encoding on .tranform()

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.38%. Comparing base (6f10c78) to head (f3d0a04).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #136 +/- ## ========================================== + Coverage 86.18% 86.38% +0.19% ========================================== Files 34 36 +2 Lines 5177 5222 +45 ========================================== + Hits 4462 4511 +49 + Misses 715 711 -4 ```

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