Added a marching cubes algorithm to retreive normals

This patch implements the Marching Cubes algorithm to approximate the normals surrounding a given voxel in the octree. It checks the 8 voxels located at the vertices of the provided voxel for their state and return the normal of the triangles given by the Marching Cubes algorithm. Averaging these normals  can give a good approximation of the orientation of the surface inside the voxel.

- The linear interpolation of the triangles vertices is not yet implemented.