fix masking issue in interpolate (#161)

* fix masking issue in interpolate

* remove and add comments

Author: J22Melody

src/python/pose_format/numpy/pose_body.py

@@ -339,7 +339,7 @@ def interpolate(self, new_fps: int = None, kind='cubic'):
         for people in points:
             new_frames = []
             for frames in people:
-                mask = frames.transpose()[0].mask
+                mask = frames.transpose()[-1].mask # takes mask from confidence value
 
                 partial_steps = ma.array(steps, mask=mask).compressed()