Replace use of deprecated collections with get_paths

[magnusuMET]

Features are now created per contour-level instead of per-polygon.

For contour_to_geojson we now split the path into subpaths ourselves, but it should be rather straight forward.

For contourf_to_geojson there are some larger changes as one path is emitted per level. Here the contours are combined by checking the handedness of the polygons, and assuming the contours follows sensible conventions, i.e. the first CCW polygon is a ring and following CW polygons represents holes.

Benchmark files had to be updated to update level-index and the color which was off-by-one if I understand the code correctly.

[magnusuMET]

This continue would not increase contour_index which might have been a bug

[jgriesfeller]

Not that I'm really able to understand the code since I'm not very familiar with the geojson format. But the code looks good and since we are using the library quite extensively, I assume we will find problems right away.