sa.plot_line now returns the width of the bin used to make the line.

This allows mt.new_moments() to extract the correct-width bin from the
simulation file.
Updates to ADESALT that follow rwidth work from previous commit
Model and data in new_moments are now moment-ized in exactly the same way.

Experience shows that this results in a 1-index offset from the way
do_line was calculating moments of the models.
Updated moment calculations to include the 4th moment.

Moment_tests got a new function to measure this moment in various models.
Fixed bug in error/moment calculations in new_moments.

This bug was only in new_moments, not ADE_Utils.ADE_moemnts. The vectors
being passed to the moment routine were getting unnecessarily fuct.