improve isosurface computation

[jbweston]

When computing the isosurface, when 4 simplex edges crosses the isosurface, we add 2 triangles to the isosurface triangulation.

Right now we add these arbitrarily (in these 3 lines) but we should add them so that the curvature is minimized.

The current state of affairs produces plots like this:

Note how the isosurface on the right hand side of the figure is not smooth.

[oroszl]

Hey @jbweston can you put up the example notebook for producing the above plot somewhere?

[jhoofwijk]

I had though about that before, but the four points which are found are co-planar, so it doesn't matter in what way you make the triangles. The roughness comes from differences between adjacent tetrahedra.

[akhmerov]

@jhoofwijk thanks for the clarification, that's an excellent point.

[akhmerov]

OTOH the isosurface seems to have artifacts... wondering how to investigate the specific origin of those...