Add Mass-Gap as new category for EM-Bright
The decision to move mass-gap classifier from the source-classification to the source-properties was postponed to O4. New classifier needs to be generated after conducting the injection campaign with injections in the gap.
Mass gap definition: 3.0 M_sun <= M <= 5.0 M_sun
[CBC chairs request from O3: @jolien-creighton @chad-hanna @ed.porter]
Edited by Shaon Ghosh