* Update for new version of thinc. Tagger still needs to be updated for new search.
* Messily fix off-by-one error in tagger.pyx
* Make tagger beam its own file, until update to new thinc beam
* Update requirement to thinc 1.3
* Upd requirements
* Rollback changes to parser for off-setting, which has been removed from thinc.
* Remove assert-not-zero from tagger
* Add typing to fill_context, for efficiency
* Change parser to use thinc.search beam implementation
* Set is_valid=False within fill_costs, to make it easier to follow the gold derivation
* Remove assertions
* Change type of index variable
* Decommision old parser beam implementation
* Inline state-fetching functions
* Inline the validity functions
* Remove error codes from beam callback function pointers
* Fix apparent off-by-one in tagger
* Use NonZeroConjFeat, not ConjFeat
* Add docstring to parser.pyx:train function.
* Add doc string for train_sent function