Percentile legend for eq_hist?

[jbednar]

For eq_hist, there are two useful ways to label the colormap ranges: as numeric values, and as percentiles. Percentiles should be linear across the visible range (if histogram equalization has succeeded), which should be simple to support (just a regular spacing of percentages), and should be simple for people to understand. Actual numeric value ranges are also important, but more difficult to construct; ideally the legend would show both.