Improve dpr conversion

[Timoeller]

implemented changes

• The key value pair "passage_id" was missing during conversion of squad to DPR.
• Added some minor improvements to the code.

open TODOs

• preprocessing of positive context (currently the whole passage is taken instead of a 100 word passage containing any of the annotated answers)
• the option for additional data besides SQuAD training set, such that hard negatives can be chosen from a larger pool of passages
• [optional] adding of no_answers as negative passages only (this change requires adjustments to the training code that always expects a positive context)

[brandenchan]

Tried it out and it seems to work well! LGTM