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
please compute a flag which fitter to pick outside of the loop.
The string manipulation here is visible in profile.