Follow-up from "Modernize MuonAssociators"

The following discussion from !414 (merged) should be addressed:

• @graven started a discussion: (+3 comments)

  Please note that will uno is assigned here, due is never assigned. I suspect this is a bug, but for now I've kept the code 'bug compatible' -- I propose to fix this in a dedicated MR to disentangle bug fixes from 'technical' changes.

In practice: create a MR uncommenting https://gitlab.cern.ch/lhcb/Lbcom/blob/ac7758051d20d32260a26d542505327dde06363c/Muon/MuonAssociators/src/MuonPad2MCTool.cpp#L168 once !414 (merged) is merged