Faster vol viewer and marching cubes

Adds a 3D viewer of the boundary of a volume (we already had volumetric visualisation). This is a simple viewer for the surface of the volume, either a set of surfels or as a set of its immediate inner or outer voxels.
Speeds up 3dVolMarchingCubes by replacing the standard digital set by a simple predicate on the input image. For instance, the isosurface of Bunny256 takes 10 times less than before.