Implement marching squares algorithm for contour line calculation

[Framstag]

We now have some rudimentary code for calculation contour lines (See #1030). We should exchange the current naive algorithm with https://en.wikipedia.org/wiki/Marching_squares (or another algorithm with the same goal) to improve the contour of the contour lines. This should allow use to generate "real" ways and/or areas with contour line style and nice border labels.