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.
I introduced some tuple formalism for the calcVariables function, in this way I bring declaration and initialization closer to each other, it's probably also easier to understand the code seeing there are three values returned by the function. I don't really like to initialize a variable with some non-sense value which will be overridden anyway, so I clearly prefer it with this tuple formalism.