Add CP::IClassificationTool + CP::AsgClassificationDecorationAlg

This is a first step in migration of IFFTruthClassifier to Athena (see https://indico.cern.ch/event/916082/).

I propose a new CP::IClassificationTool interface (thanks @jburr for the idea) to get the classification as int. A companion CP algorithm is also added to decorate this on a particle.

Note that this is not fully functional yet as there is no tool that uses this interface, but I wanted to keep the amount of changes small. I supposed this might start some discussion, so I start with an interface proposal.

/cc @krumnack @christos @tneep @oducu