BPHY19 derivation placeholder for di-muon analyses with photon conversions
This merge request adds BPHY19 to the derivation framework and supplies a placeholder BPHY19.py file.
The BPHY19 derivation format is intended for di-muon analyses with photon conversions like radiative chi_b/c decays.
The development is followed up in https://its.cern.ch/jira/browse/ATLBPHYS-103 .
cc: @abarton