Small update of FastPV sequence

Small updates of the FastPrimaryVertex code and JetVertexChecker, presented at TSG meeting:
https://indico.cern.ch/event/338610/contribution/1/1/material/slides/0.pdf

In both Producer is possible to maintaing the "old" method, with the options:
- "newMethod" in JetVertexChecker
-  "m_ptWeighting" in FastPrimaryVertexWithWeightsProducer
better use std::abs()  instead of fabs (well, consistently in the code).
... and should I mention the "magic" number too (used twice).