Get predictions with standard errors into data frame for use with geom_smooth
# S3 method for polr
predictdf(model, xseq, se, level)object returned from polr
sequence of x values for which to compute the smooth
if TRUE then confidence intervals are returned
confidence level for confidence intervals
predictdf.polr is used by xgx_geom_smooth when method = "polr"
to calculate confidence intervals via bootstraps.