Draft: Merge branch 'mwilkins/dielectrons' into 'master'
add decay files for dark-photon/true-muonium search in pi0/eta -> gamma (A` -> e+ e-)
See merge request !1068 (merged)
(cherry picked from commit 8d44c78b)
57236336 add decay files for dark-photon/true-muonium search in pi0/eta -> gamma (A' -> e+ e-)