[trackingdata] avoid div by zero on center of gravity estimation

fixtracks/utils/trackingdata.py

@@ -199,8 +199,11 @@ class TrackingData(QObject):
         scores[:, np.setdiff1d(np.arange(scores.shape[1]), nodes)] = 0.0
         weighted_coords = self.coordinates(selection=selection) * scores[:, :, np.newaxis]
         sum_scores = np.sum(scores, axis=1, keepdims=True)
-        center_of_gravity = np.sum(weighted_coords, axis=1) / sum_scores
-        return center_of_gravity
+
+        cogs = np.zeros((weighted_coords.shape[0], 2))
+        val_ids = np.where(sum_scores > 0.0)[0]
+        cogs[val_ids] = np.sum(weighted_coords[val_ids], axis=1) / sum_scores[val_ids]
+        return cogs
 
     def animalLength(self, bodyaxis=None):
         if bodyaxis is None: