Plot of test error rates

[haziqj]

...over time/iterations.

[haziqj]

I plan to make a wrapper function to combine fitting, plot of lower bound, plot of test error rates and plot of predicted values all in one screen. Perhaps a shiny app of this would be nice too.

[haziqj]

Instead of the original plan, I added option train.samp, so iprobit knows to split the data into training and test samples. At each iteration, the fitted values and error rates are calculated, and this is plotted over iterations.