Qg tagger update nov2014

Update the QGTagger.cc file with:
- thread-safe implementation of jet corrections on the fly (based on https://twiki.cern.ch/twiki/bin/view/CMSPublic/FWMultithreadedConsumesJetCorrectorMigration)
- introducing pt > 1 GeV cut on neutral particles for axis2 and ptD calculation (improves slightly the stability versus pile-up)
- make the dz and dsigma cuts optional ("useQualityCuts" boolean, false by default): the impact of these cuts on the performance of the tagger seems to be very small on MC. Without them we can easily apply the tagger also on miniAOD, and therefore we switched them off at the moment. Though the possibility to switch them back on later is kept, because there is a possibiltiy they give better data/MC agreement.
There is no need to call sqrt here: just square the cut value below (d0*d0 is cheaper than sqrt(sigma))