High Granularity Optimization

speed up various critical packages by using Ofast option
as described in
https://twiki.cern.ch/twiki/bin/view/CMS/HighGranularityCMSSWOptimization
and discussed in April in some Reco/Offline meeting

a small regression is observed in primaryVertices (greatly mitigated w/r/t the very first attempts).
This "cascades" in general tracks and conversions as primaryVertices are used in the seeding
this line is perfectly legal and does what is supposed to do (avoid a division by zero)
std::numeric_limits<double>::epsilon() , as you know well, is just the ULP for 1.  and therefore pretty irrelevant for any other value.