Likely remove Parts' tokens

[juanmirocks]

A list of tokens was added by Ashish for the Rostlab/relna project. However, lists of tokens are already incorporated into sentences. Therefore, this might be redundant and confusing for nalaf users.

• Investigate if tokens are needed in parts / sentences / entities (see General list of tasks of merging nalaf, nala, relna #162)
  -> so far left in Entity as it's extensively used -- unnecessary in Part as it's already in its sentences
• ...take necessary steps
  -> remove tokens from Part

[juanmirocks]

[05/02/16 12:31:40] Aleksandar Bojchevski: Any idea why the Part object has:
self.tokens = []
[05/02/16 12:32:07] Aleksandar Bojchevski: we never store tokens there... the tokens are part of 
self.sentences = [[]]
[05/02/16 12:34:45] Aleksandar Bojchevski: seems to be added recently
[05/02/16 12:38:07] Aleksandar Bojchevski: https://github.com/Rostlab/nalaf/commit/8a6e3783e987f0616a0cb6533d8df30c0a255d66
[05/02/16 12:38:18] Aleksandar Bojchevski: apperentaly it was added by ashish
[05/02/16 12:38:45] Aleksandar Bojchevski: hmm... might be an artefact... we should check relna
[05/02/16 12:46:17] Juan Miguel Cejuela: let me check
[05/02/16 12:46:35] Juan Miguel Cejuela: What I can recall is that he needed tokens in entities
[05/02/16 12:48:18] Aleksandar Bojchevski: that I know
[05/02/16 12:48:22] Aleksandar Bojchevski: but not in parts
[05/02/16 12:49:05] Juan Miguel Cejuela: Yes, in his file parsers.py I see that he is also using tokens in parts