Leunbach Test Equating // Implements the Leunbach test equating method, following the 'DIGRAM' software written by Svend Kreiner. Both direct and indirect equating are available, with parametric bootstrap standard errors and diagnostic statistics including the Goodman-Kruskal gamma test and orbit analysis for person fit. See Adroher et al. (2019) doi:10.1186/s12874-019-0768-y for details of the method.