V0branch - modifications to V0Fitter (V0Producer) for PU

1) added new cuts and modified old  default values to purify the V0 collection in presence of PU (optimized for PU25). for slides from a tracking POG meeting see:
https://indico.cern.ch/event/337839/
https://indico.cern.ch/event/337839/contribution/3/material/slides/0.pdf
2) deleted code that was no longer relevant to the module
3) rearranged code to apply selection cuts sooner to help speed the execution of the module
I would've just made a vector of  ```const Track*``` and dealt with them.
Since you later need the index as well to make a TrackRef when the pi/p candidates are made,
I'd probably chose a vector of ```pair< const Track*, size_t> ```

Refs a re needed for persistency/storage and carry quite a lot of baggage with them

The number of lines in https://cmssdt.cern.ch/SDT/lxr/source/DataFormats/Common/interface/Ref.h may give an idea that it's not a simple class