Displacements in fitting result

This PR adds the `displacements()` method in the `FittingResult` class. This method returns the displacement between each iteration's shape and the precious iteration's shape. It can either return the mean, median, min, or max displacement (point-to-point euclidean distance) or the displacement of a specific landmark point.
not sure I like this API here with the string interpolation..

I think it might just be better to provide the user with the raw displacement values, and then they can do what they want:

```python
displacements = [np.linalg.norm(s1.points - s2.points, axis=1)
                 for s1, s2 in zip(shapes, shapes[1:])]
```