Extending the GaudiConfig2 property sematics with a few ATLAS-specific cases

The configurables coming from GaudiConfig2 use 'semantics' for property types. The python file added here adds semantics that we need for ATLAS. So far these are:

• appendListSematics: An extension of the general SequenceSemantics that re-defines the merge-method so that the lists get concatenated
• SetSemantics: An extension of the general SequenceSemantics that re-defines the merge-method so that the duplicate list-items are unified (like in a set)
• HandleSematics: Makes sure that ReadHandle,WriteHandle, etc are treated like strings.

This is just a starting point, more special semantics are needed.

Adding @clemenci who helped me getting this going. Marco, I would prefer if we could extend the SequenceSemantics w/o having to repeat the template parameter in the fifth parameter of the Gaudi::Property constructor.