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
Any non-private (without `_` at the start of the name) function should have a complete docstring. This one maybe should be renamed `_mix` to make it clear it won't be exposed to users. Even private functions should have a minimal descriptive docstring, so something like:
```
def _mix(a, b, t)
    """Mix b (with proportion t) with a"""
    return (1.0 - t) * a + t * b
```