Follow-up from "WIP: Functor for smog di-muon line"

The following discussion from !2199 (merged) should be addressed:

• @olupton started a discussion: (+2 comments)

  A word of explanation: I didn't foresee this use-case, so the loop_mask() method is only added when I make the proxy type that derives from the individual container proxies. This is a hack that gets to it by CRTP using a magic name, but I think we could just define it here instead so it can be used directly (this->loop_mask()) in cases like this.