Add D02pipipi0 with preliminary AMPGEN model

Adds the DecFile: Dst_D0pi,pipipi0=TightCut,AmpGen.dec # EventType: 27263474

This is used for studying efficiencies for CPV studies via amplitude fitting in this model.

The relevant LbAmpgen generator is available after Gauss/v56r2.

The local test yields a generator efficiency of 54/3803 = 1.42%.

Untracked files: v30r999/ v999r999/