Add a pt cut to b-tagging track association

This lets us only associate tracks to jets if the jets are over some pt threshold. Should change nothing at all, since the default threshold is zero.

But this should be useful to make our b-tagging more robust in the future. In particular, with delta-R based association, the track association for all jets is sensitive to the lowest pt jets in the event, which means that moving the jet building threshold has global effects. Our colleagues, perhaps spoiled by IRC-safe jet reconstruction and in particular anti-kt, are often tempted to change the jet building threshold on a whim, often to save space or CPU. If we set a lower bound for track association, we should be able to insulate ourselves against these kinds of changes.