Break plot_obs_pred into two functions

The input part is separated from the plotting part to make the function
more generic. All plot_obs_pred_* functions will have to be modified
accordingly.
Adjust plotting functions

All plot_obs_pred_* functions are modified to be consistent with the
previous modification on plot_obs_pred.
Add function to break down results by site

Add function plot_four_patterns_single to create the four pattern figure
for each plot individually and save all figures into one pdf.
Fix inconsistencies in file name for *_obs_pred_rad.csv (previously
*_obs_pred.csv).
Minor modifications to plot_fig1

Change variable in panel B from DBH to DBH**2.
Add legend to panel D.