use DFCommonJets QGTagging variables

Update the BDT q/g tagger to use the DFCommonJets_QGTagger variables instead of calculating/retrieving them itself.
Also add the option for the user to choose between the DFCommonJets_QGTagger variables or calculating them, in case they are not available.

One problem is that in some derivations not all ghost associated tracks are stored, so we need to check if that's the case (see comments). This has the consequence that is NumTrkPt500 (i.e. nTracks) is not available an error is raised.

Other smaller changes:

• move calculation of variables to separate function
• make getJetProperties private
• fix unchecked successes warnings in test_JetQGTaggerBDT.cxx
• fix error with TChain in test_JetQGTaggerBDT.cxx (this error did not occur in older versions(?))

@bnachman @woodsn @kfiekas @meehan @dguest @sschramm