MiniAOD extension for quark-gluon tagger

The QGTagger.cc plugin is updated to work also on pat::Jets and pat::PackedCandidate.
The QGTagger class is now a templated QGTagger<jetClass>, so that instead of the original QGTagger plugin, we have a QGTaggerPFJets and a QGTaggerPatJets defined.
The updated cfi fragment shows how it can be used in different situations.
The code is also a bit cleaned up to be more readable.