Backtracking improvement

[YifanC]

• use non-zero value (choose a value that cannot be valid value) for filling value:

  ndlar_flow/src/proto_nd_flow/reco/charge/calib_prompt_hits.py

  Line 227 in 114b59a

  back_track = np.full(shape=packets_arr.shape,fill_value=0.,dtype=self.hit_frac_dtype)

• propagate event_id from the larndsim truth propagation.