Small update of FastPV sequence - backport 72X

Backport in 72X of https://github.com/cms-sw/cmssw/pull/5667
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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


there should be a check for division by zero for "empty" events