Need to update the MBTA reference

The 2016 reference [Adams et al. CQG 33, 175012 (2016)] for MBTA is outdated. Please update to DOI 10.1088/1361-6382/abe913 [Aubin et al. CQG 38, 095004 (2021)]. This will be consistent with the references of the emfollowup usersguide that was recently updated.