Grain plot task: enrich exported maps

Right now, what is exported is a series of polygons and color values.

In addition to the color values, please also export the numerical height value of this contour.
In addition to exporting the final contours, our users would like to have the map of z-value as function of the two motor positions, such that they can choose their own contour heights when they re-create the figure.

Edited Jun 08, 2024 by Wout De Nolf