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
I guess we could have something in the function docstring like:
```
    Notes
    -----
    GLSL equivalent code:
    ...
```
And then parse for the phrase `GLSL equivalent code` when looking for the GLSL in the docstring. @almarklein is that what you had in mind?