Strong question words

Fix https://github.com/ProjetPP/PPP-QuestionParsing-Grammatical/issues/58 (a very useful bug, the question parsing in 23% better :)

What we do: 
  - a question word is strong if the info it contains is not elsewhere in the sentence (ex: `where is Paris`, if you remove `where` you no longer know that you are looking for a place)
  - until now, an extra triple depending on the question word was added only if the simplified tree was of depth 3 (typically : `ROOT -> is -> Paris` = `(Paris,location,?)`)
  - now, we also add an extra triple if the depth if >3 and the question word is strong (`where is the capital of france` -> `((france,capital,?),location,?)`)