DistilledPFOCreator Processor

Purpose: Create a PFO collection consisting of the set of ReconstructedParticles that is the union 
         of the PandoraPFOs collection and the GammaGammaParticles collection (consisting of unique mass-constrained gamma-gamma combinations) 
         while avoiding any double-counting of GammaGammaParticle constituents.

Method:  Read in PandoraPFOs and GammaGammaParticles. 
         Create a subset collection taking care of double counting.
         The DistilledPFOs collection can be used instead of PandoraPFOs for jet energy reconstruction.

An example xml steering file is examples/example_DistilledPFOCreator.xml

Graham W. Wilson, 15th October 2015.