Sc 50 disk backed operations

[ArneDefauw]

PR fixes issue when spatialdata object is disk-backed, see #90.

Fix was to do the following, e.g.

            if sdata.is_backed():
                sdata.write_element(output_layer)
                sdata_temp = read_zarr(sdata.path, selection=["images"])
                del sdata[output_layer]
                sdata[output_layer] = sdata_temp[output_layer]
                del sdata_temp

if spatialdata would support write_element( reload=True ), we should adapt our code, and use the reload parameter.