Add functions

1. get_iisd_pred_dbh2 to obtain predicted dbh2 for iISD.
2. get_obs_pred_iisd to write obs and pred iisd to file.
3. plot_four_patterns_ver2 to plot the four patterns. ISD and iISD are
plotted in ranked form.
4. plot_four_patterns_single_ver3 to plot the four patterns for
individual sites in the same form as plot_four_patterns_ver2.
5. bootstrap_rsquare_loglik_SAD to obtain R^2 and log-likelihood of
bootstrapped samples of SAD.