Add DecFile for K+ -> pi+ pi- pi+, with two pions in acceptance
Add DecFile for K+ -> pi+ pi- pi+, with two pions in acceptance and minimum p/pT, for efficiency studies. The kinematic cuts should be softer than the ones in the stripping line: https://gitlab.cern.ch/lhcb/Stripping/blob/run2-patches/Phys/StrippingSelections/python/StrippingSelections/StrippingCalib/StrippingK23PiForDownstreamTrackEff.py