Add a radial option to analyze.distribution

[k034b363]

Is your feature request related to a problem? Please describe.
In some situations, like identifying bright embryos within seed xray images, it would be great to have a radial version of analyze.distribution, such that the output would describe the count of nonzero pixels (or average value?) in bins that specified distance from the center of a labeled object.

Describe the solution you'd like
Either a "radial" category for the direction parameter in the existing analyze.distribution function or a separate analyze.distribution_radial function that does what is described above. The output would be similar, a histogram for visualization and outputs for either count or average value in bins. Because labeled objects will not be perfectly round, there should probably also be a cutoff option for where the distribution stops to account for strange things at the edges. Also, the count would probably need to be a percentage, since bins will be uneven (there will be more pixels in farther bins).