Area-based contours

[settwi]

Describe the feature

For X-rays (and radio I think?) we like to put contours on images based on the "area contained." The current GenericMap.contour method, when given percentiles, puts those contours relative to the max value in the map.

If we want to do it by area we have to do a little bit of shenanigans with sorted arrays, like in this gist. We could extend the example to N-dimensions, too.

This would be a pretty small addition to the contour code but probably make people happy!

Proposed solution

See the request! There is an example. Up for debate if we want N-dimension compatibility and how to add units in logically.