set parent_length from without Dna_7::set_indiv(Dna* dna)

src/aevol_modify.cpp

src/libaevol/7/Dna_7.cpp

src/libaevol/7/ExpManager_7.cpp

...ost_treatments/aevol_post_ancestor_network_knockout_7.cpp

src/post_treatments/aevol_post_ancestor_stats_7.cpp

src/post_treatments/aevol_post_create_csv_7.cpp

src/post_treatments/aevol_post_create_eps_7.cpp