Perform histogramming for density-plots in equi-angle cubed-sphere co…

[MACarlsen]

Resolves #441 by performing the histogramming in six separate charts, corresponding to six cube faces, in gnomonic equal-angle coordinates. This avoids the coordinate divergence at the pole, which is unavoidable in any single-chart map of the sphere.

Smoothing is handled by diffusion on the cube surfaces and across edges.

I might interpolate the smoothed histograms onto an longitude-latitude map to avoid down-stream changes. Remains to be seen.