Support for Marching Cubes with Mesh Displacement

[runevision]

This pull request adds support for mesh displacement, a technique which reduces the triangle count by half compared to Marching Cubes on its own, and makes for more regular proportioned triangles.

Based on the paper "Mesh Displacement: An Improved Contouring Method for Trivariate Data" http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.49.5214

The technique was used in the videogame Spore in the Creature Creator. See: http://chrishecker.com/My_liner_notes_for_spore