notebook 13 - improve reprentation output of rasterized rivers and burned dem

[stijnvanhoey]

temp = clipped.to_dataset(name="dem")
temp["rivers"] = xr.DataArray(data=img, coords=clipped.coords)

also the viz can than use xarray operators

[stijnvanhoey]

See #65

ds_zwalm = clipped.to_dataset(name="dem").squeeze()
ds_zwalm["river"] = xr.DataArray(data=river_raster, coords=clipped.coords)
ds_zwalm["dem_burned"] = ds_zwalm["dem"] - ds_zwalm["river"]*20