[FIX] API PUT ngram: new param 'testgroup' can test not only if ngram exists but also if ngram has a mainform

[FIX] new ngram indexing: fix cases where ngram was already partially indexed (ex: if extraction was only correct in some documents) (todo: change bulk_insert_ifnotexists for cases like this)

add corpus: reactivate load button for file corpus via minimalist a posteriori validation checks in checkReady function (en attendant mieux :) )