[DRAFT] added agumentation yielding evenly distributed classes

[TiMauzi]

What does this PR do?

Summary

This PR adds a new augmentation method called augment_text_with_ids_evenly which yields a similar output to augment_text_with_ids, but always returns equal numbers of strings per ID. The argument additional_examples can increase the number of augmented samples further beyond the maximum number of examples over all classes. The argument perfectly_even asserts an exactly equal number of samples per class, but the augmentation will take slightly longer.

Additions

• Added augment_text_with_ids_evenly.

Checklist

• The title of your pull request should be a summary of its contribution.
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• All public methods must have informative docstrings that work nicely with sphinx. For new modules/files, please add/modify the appropriate .rst file in TextAttack/docs/apidoc.'

[TiMauzi]

Please excuse the poor commit message, since it used to be "added augment_text_with_ids_evenly".