Add coordinates for "y2" (topographically corrected data) in export

[erikmannerfelt]

Currently, there is no easy y-coordinate for the topographically corrected data. The dimension y2 should have elevation associated with it.

The y2-coordinate should technically be possible to calculate from the x/y coordinates (elevation and depth), but I honestly don't see a trivial way to do it, so it'd be much better to just export the y2 elevation instead.

[erikmannerfelt]

In xarray, this is approximated by:

data["elevation_topo"] = ("y2",), np.linspace(data["elevation"].min().item() - data["depth"].max().item(), data["elevation"].max().item(), data["y2"].shape[0])[::-1]