td-idf of terms, add synonyms from forum train set

[reality]

after #1 calculate word frequencies wrt length of threads using td-idf. doing this iteratively (removing terms directly expressed in the ontology each time) will inform you on synonyms and new terms to add, until you have coverage of all relevant terms in the ontology.

looks like the td-idf isn't in nltk directly, but you can use it to calculate the measure: https://nlpforhackers.io/tf-idf/

It looks like scikit-learn does have the functionality directly though: https://www.bogotobogo.com/python/NLTK/tf_idf_with_scikit-learn_NLTK.php