see fig1dat_init.do.
for figure 1, scenario 1, SJ article: Janes, Longton, Pepe.       Accommodating covariates in ROC analysis. 2008.
Simulated data for a continuous marker y, binary outcome d,       and binary covariate z.  Marker values vary with z, but the       discriminatory accuracy of the marker (i.e. the ROC curve) is       the same in the covariate-specific populations.  In scenario 1,       there are different proportions of cases (d=1) and controls (d=0)       in the two covariate groups.  In scenario 2, the case-control       ratio is constant across the covariate-specific populations.