Analytical and plotting changes

Analytical changes:
1. For get_obs_pred_frequency, change log bin width for dbh**2 from 4 to
1.7 to increase number of bins.
2. Fix minor bug in obtaining species-level average dbh**2 in
get_obs_pred_intradist.
Plotting:
1. Change the orientation of Fig. 3.
2. Add minimum bound for x-axis in plot_obs_pred so that axes are
consistent.
3. Add function plot_four_patterns_single_ver2 to create another version
of the plot-level figure.

All changes are pending test.