Draft: Counter alg

This adds a Counter algorithm for any xAOD::IParticleContainer and writes it to the the eventstore.

cofigurable via python like:

cfg.addEventAlgo(
            CompFactory.HH4B.CounterAlg(
                containerInKey=<xAOD container name>
                ),
                outKey="myOutKey",
            )
        )

from any following c++ algorithms you are then able to

SG::ReadHandle<int> countReader("myOutKey");
int numberOfMyObjects = *countReader;

related to #21 (closed)