Add low-pT working point on the same footing as high-pT in muon quality variable

This MR adds the functionality to store the low-pT working point decision in a bit in the muon quality variable. This should be a straightforward extension of what is already done for the "ID hits" and high-pT working point decisions. The corresponding set method in the MuonSelectionTool has been uncommented.

Tagging @nkoehler, @gartoni, @szambito