Implement Cushioned Treemaps

[ryanaslett]

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The "Cushioned Treemaps" option is not an actual cushioned map that follows the algorithm as spelled out in http://www.win.tue.nl/~vanwijk/ctm.pdf . It's currently just a gradient that doesnt really offer any additional value over a flat map. The formula presented in the cushioned treemap paper gives the viewer a high level impression of the underlying hierarchy.

It would be pretty awesome if this library supported the actual algorithm that generates those gradients (for examples, see windirstat/disk inventory X/ or Kdirstat - they all implement the algorithm).