Consolidate dump-single-btag and SingleBTagAlg

This adds a function that does everything SingleBTagAlg and dump-single-btag do. The idea is to reduce the amount of duplicated code.

I ran a diff of the outputs from dump-single-btag before and after this change. The only difference I noticed was that the jet rank changed. This might actually be a bug in the previous implementation: the jets were not sorted previously in dump-single-btag whereas now they are sorted by descending pt. I didn't see any changes in the Gaudi-based builds, probably because we were already sorting jets by descending pt there.