Minor fixes to make_distance_comparison_plots.py

Several minor fixes to make_distance_comparison_plots.py to improve plot quality:

- added auto-sizing of distribution plots via ```--distribution_width``` (this is the new default). Previously a hard-coded plot width of 0.5 was used, which didn't scale well to different inputs
- better handling of numeric label types (via ```-a numeric```) with very large or small ranges (e.g. elevation). x-axis units are now scaled to ```[1, (number of data points)]``` instead of using the original numeric range
- ```--group_spacing``` has been removed in favor of the new auto-sizing feature
- in some circumstances, ```-t box``` yielded a boxplot that was squashed against the left edge of the plot. This has been fixed