refactor ir index: class instead of globals

* use a class instance for IR index instead of functions with globals
* refactored IR index and chunk matcher
move kwd chunk matchers to proper place

* the whoosh folder was moved into entity_matchers.kwd_chunks
* renamed KWS/entity_matchers/result_fields_by_chunks.py into KWS/entity_matchers/kwd_chunks/chunk_matcher.py
  - so now it's nearby the related IR functions based on whoosh