* Improving docs
* Update doc strings
* More docs work
* More docs work
* More docs work
* Improve module docstring
* Struggling with arbitrary attr access...
* Fixed group_by, removed idea of general attr_of function.
* Add overview doc
* Further improvements to spacy docs, tweaks to code.
* Move en_ptb to ptb3
* Move en_ptb data
* Fix ptb3 module
* Fix import for ptb tokenization test
* Replace the use of dense_hash_map with Python dict
* Moving to Word objects in place of the Lexeme struct.
* Moving to Word objects in place of the Lexeme struct.
* Refactor around Word objects, adapting tests. Tests passing, except for string views.
* Tests passing for new Word object version
* Add Word classes