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
My comment was only about this block, although looking more at the rest of the code this may be not the worst part (there is an nTracks**2 loop over trackRefs later).
The point is to operate with units that are as close to the variables that you need as possible. 
Ref requires an unnecessary roundtrip of navigating to the place where the collection is and then navigating to the object, and you repeat it multiple times. Even if it were super-optimized and quick, it's more straightforward to not incur the extra overhead.