Improve contour drawings

[x] Color range should be fixable
[ ] bilinear interpolation should be up (I think - at least the current method leaves blank points at the top of the map)
[x] Try rendering the geojson as a vector layer, to see if that improves performance
[x] Color range captions should be colored according to background to be visible
[x] Hovering on a polygon should highlight the color in the color range, and vice versa
[ ] Toggle specific bands by clicking patches on the color bar
[ ] Changing depth should result in re-calculating the color bar
[ ] Should be an 'auto' button to re-calculate the color scale at a particular depth

SAEON / somisana