Always save tau scale factors with systematics + fix tau handle
Follow up to !363 (merged)
The point of tree_flags.write_object_systs_only_for_pt
is to skip the impact of systematics on 4-vector components other than the pt
. Scale factors on the other hand will need to be used with systematics variations in all cases.
Also fixes the tau handle used to retrieve the scale factor.
FYI @jdegens
Edited by Thomas Strebler