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.
better initialize all values immediately, not rely that something somewhere will set them.
Currently, all of these (pt, ptD, axis2, and mult) are eventually always set, but it takes some time until it happens.