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 should be a single-line description of the class. If you need to be more descriptive, separate it into a single-line description plus more two lines later, like:
```
Display isocurves of a triangular mesh

Something more descriptive if it is necessary. This can
be multiple lines if necessary.
```