new visuals/isocurve for tri mesh

I add a new visual to show iso line on different level on triangular mesh with data at vertices. Possibility to choose color or colormap to color the isoline
This example no longer works for me because of this import. Also, examples should not use private modules (those preceded by underscores). If you need something from a private module, we should discuss making it public. But I think what you need is probably available publicly now.