Add givenness approximation based on semantic measurements

[Diego Palma]

Currently we support givenness metrics, based on pronoun patterns. An estimate that might help for some complexity assessment tasks is using givenness based on a vector space model, as depicted in the following paper:

https://escholarship.org/content/qt87n81224/qt87n81224_noSplash_9e003a08316eb6817c3aeda212d9ae27.pdf

Implement this feature.