x_values are now scaled into range.

Still need to do some more testing.
Added auto distribution width calculation.

Removed --group_spacing from make_distance_comparison_plots.py. Still need to add/fix unit tests.
Updated unit tests.
Cleaned up code a bit and made it more efficient.
Updated ChangeLog.
Merge pull request #846 from jrrideout/mdcp_fix

Minor fixes to make_distance_comparison_plots.py