WIP: remove_all_landmarks convienience method, quick lm filter

adds `.remove_all_landmarks()` API to `Landmarkable` to safely remove all annotations. Also adds convience filter methods on landmarks, which are useful with `menpodetect`. For instance:

```python
>> print(list(group_photo.landmarks.keys()))
frontal_face_00
frontal_face_01
frontal_face_02
PTS
ground_truth

>> print(list(group_photo.landmarks.keys_matching('frontal_face*')))
frontal_face_00
frontal_face_01
frontal_face_02
```