pick out entity chains

[erindb]

export caveman/chain data for every document
from this, pick the subset of chains i used before

[erindb]

the subset of chains i used before, i meant to pick out only sentences where the referent was subject/object/head-of-prepositional-phrase, but i was a little imprecise. so in experiment 2, there were some extra sentences. so i'll use a slightly different set of chains in experiment 3.