read_watersurfaces(): reorder levels of depth_class

[florisvdh]

Up to now, the depth class levels in the return value of read_watersurfaces() were alphabetically ordered, but this results in an inconvenient order.

Since depth classes differ between different versions of the data source, this requires a bit more investigation in order to choose the easiest way to implement this in read_watersurfaces().

Based on this comment: inbo/n2khab-preprocessing@265fbe0#r151058765