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
Function and class descriptions should be a single line. If you need more space, do something like this:
```
"""Generate an isocurve from vertex data in a surface mesh

Uses a marching squares algorithm to generate the isolines.
"""