Landmarks fixes

Just some updates that help ease the transition for the changes that @jabooth has been making recently in order to break the cyclic/weak references!

  1. Change the indexing of landmark group so that it returns a `PointCloud` (instead of having to use `lms`)

    ```
    pcloud = img.landmarks['group'].lms  #  The whole PointCloud as before
    pcloud = img.landmarks['group']['label']  # A subset of the original PointCloud
    ```
  2. Remove the group label from landmark group since we didn't use it anyway
  3. Update viewing to be more useful
    a. @jabooth already added the `view_landmarks` method. It now has more kwargs that allow you to filter
        the landmarks that you are viewing. So now, we do:
        ```
        img.view_landmarks(group_label='ibug_68_points', with_labels=['reye', 'leye'])
        ```

You can continue to view landmarks without the underlying object, and they can be filtered in the same way. This just seems more natural and not as cumbersome as it was in master at the moment!