compute and store parent pdg (requires SusyNtuple b1c8fd9)

A small improvement to store the pdg of the parent from the mc truth.
(needed for Daniel A.'s study on the ttbar truth).
Tested on `McAtNloJimmy_CT10_ttbar_LeptonFilter` and `Herwigpp_UEEE3_CTEQ6L1_DGnoSL_TB10_050_100_100_2L`.