move `walk(Particle const&)` from Rec to LHCb
move walk(Particle const&)
from Rec (specifically, JetUtils) to LHCb so it can be re-used in other places.
note: Rec!4082 (merged) removes the code from Rec, and picks it up from here instead - so this can be merged independent of Rec!4082 (merged), but Rec!4082 (merged) depends on this MR.
Edited by Gerhard Raven