SED from Cube code

Made a pull request while I work on this in case you have comments.
This is brittle code ... I'm not sure if this takes the whole image or if the edge pixels are removed.

If you change the `cube_sed` function following my suggestion above of only taking a `mask` as input,
you could simply remove this code if you want the full image.

And if you want a rectangular sub-part, you call a utility function that makes this mask.
I don't know if it exists, but it could be
```
lon_lat_rectangle_mask(lons, lats, lon_min=None, lon_max=None, lat_min=None, lat_max=None)
```
where the default `None` means no limit.