brem functors tests
Goes with Rec!3233 (merged) and LHCb!3894 (merged). See Rec!3233 (merged) for more info.
Adds tester/monitor of DTF mass constraint dielectrons to test brem covariances.
Also fixes duplicates due to wrong decay descriptor (see link), as this causes bremsstrahlung test to fail too. Further fixes in this regard in !2101 (merged).
Closes LHCb#237 (closed)
Edited by Maarten Van Veghel