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.
this isn't quite correct (correlations are not considered), but I could guess it's good enough for the purpose
(although in cases where vertex is quite uncertain "along" the track, then the formula will be quite incorrect)