Fairlie Decomposition for Nonlinear Models (Logit Models) // Performs the Fairlie decomposition for nonlinear models (logit models). This function breaks down the difference in average outcomes between two groups into the part explained by group differences in observable characteristics and the unexplained part (Fairlie 1999 doi:10.1086/209914; Fairlie 2005 doi:10.3233/JEM-2005-0259).