MuonNSWAsBuilt: Fix cppcheck warning.
Change Element::addDaughter to return the pointer added. Otherwise, cppcheck may warn about using a dangling pointer. (That's not really the case, but realizing that requires knowing what addDaughter is doing.)
Change Element::addDaughter to return the pointer added. Otherwise, cppcheck may warn about using a dangling pointer. (That's not really the case, but realizing that requires knowing what addDaughter is doing.)