Bug: sts_v20a was missing from repo. sts_v20a is the same as sts_v19a
only that it is shifted by 40~cm downstream consistent with a change
of the global coordinate system from old target position to the center
of the magnet. Translation is not done via a seperate matrix included
with the TFile, instead internal via a transaltion between the top
and sts_v20a volumes.