Add convenience property to get the coordinates from the `Image` object

This can be a shortcut for those who do not want to deal with the multiple lines of getting the shifts. For example instead of:

```
shifts = d.compute_offset(ref)
# do something with shifts.x and shifts.y
```

it could be:

```
x, y = d.compute_offset(ref).shifts
```

or even:

```
x, y = d.compute_shifts(ref)
# which calls compute_offset(ref).shifts under the hood
```

TODO: better naming
