Account for pointless average z-beamspot "correction"

Changes the default parameter to enable this by default. Could remove the ability to enable / disable but for the time being it is better to leave it as a flag to allow for studies of the potential impact of disabling / enabling.

Might change the rate of some muon iso chains, in which case the references will be updated. However, if the MC samples used for the tests always have the beamspot average z at 0, then no differences should arise.