Feat/kb concept extraction

Proposition for a generic interface for knowledge-based concept extraction based on the proposal made in !62 (merged).

I also created tools to manipulate candidate terms in commons.candidate_terms_tools.py. Those tools contain an adaptation of the recursive merge on synonyms, a function to merge a set of candidate terms and an adaptation of the "have synonyms" method.

I propose a candidate term method, to_concept(), to create the concept corresponding to the candidate term. Also, propose a method for the Enrichment class to merge an enrichment with another. I believe those methods will be used in different places, so attaching them to the classes makes sense.