CaloTrkMuonScore - Avoid creation of bin vector

Hi,

this is a minor clean-up of the CaloMuonScoreTool. The eta and phi bins of the calo cells were determined by constructing an auxillary bin-vector first instead of directly calculating it as it is done now. I also thought that the getMedian method introduces an unnecessary copy, but given that it sorts the elements internally, this one is unavoidable not to break the physical meaning of the variables.

Let me explicitly tag, @rwoelker who's the author of the tool and might want to check the changes as well.