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 have made all the changes you suggested with the exception of this one about the TrackRefs - I need a little clarification about this... Are you saying do not use them anywhere? or just in the block which is seen here in the attached image? What is the reason for not using them (are they costly when I use the "->" operator?)

In the attached block of code I add the TrackRefs to vectors which are then looped over to find good track pairs, is this part OK?

![trackcollection](https://cloud.githubusercontent.com/assets/6392809/4762055/6437464e-5afd-11e4-8993-cc40e07114dc.png)