Weight transformation fails if other event weights are present

[ajgilbert]

E.g. if systematic variations are enabled in the MG cards. Would be best to make the weight transformation code smarter and have it pick out just our weights.

[ajgilbert]

@fstaeg may have run in this problem too (with extra weights that appear when True = use_syst set). One option would be to always override this setting somewhere in the workflow to make sure the weights are switched off. Or as above, make the weight transforming code smarter in identifying the range of weights we need to operate on.