Add generic Pi0ToGammaGamma ThOr line

Add very simple Pi0ToGammaGamma line to generic ThOr lines to test combinations with neutrals. Based on !1098 (merged)

Reference changes expected for Hlt2Conf.test_hlt2_reco_plus_thor_selections{_fastest}.

Partially addresses #407

Includes !1434 (merged) (for testing convenience)