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
explicit string is not needed here
```
tkChi2Cut_ = theParameters.getParameter<double>("tkChi2Cut");
```
is good