MuonGeometry Svc/Cond unify code

This follows !59380 (merged) !59325 (merged) !59277 (merged)

Same idea

• We create an Impl clas.
• We move all the common things to it .
• Note that is NOT new code but copy paste of the "common" code
• So it has the same "issues", but now in one place not two (so potentially easer to solve in the future?)
• So we end up with a single 2800 lines impl, rather than 2x2800 ones ...
• Here I went back and tried to unify a bit more the InertMaterial /Station builder treatment
• At large so far in case of doubt I went the way the Cond does things.

If this succeeds (tests prb are the only way to be sure) then we might need to do a small final tidy in a follow up MR. Prb after the similar efforr by @jchapman on Inner detector also progresses more we will need another look.