Fix operator precedence bug in v_rel computation

Due to Python operator precedence, v_i - v_j / ||v_i - v_j||
was computing v_i - (v_j / ||v_i - v_j||) instead of the intended
(v_i - v_j) / ||v_i - v_j||. Split into two steps to make the
order of operations explicit.

Author: chraibi

pedpy/methods/foo_calculator.py

@@ -111,12 +111,11 @@ def compute_avoidance(
         - shapely.get_coordinates(matrix.point_neighbor)
     ) / distance[:, np.newaxis]
 
-    v_rel = shapely.get_coordinates(matrix.velocity) - shapely.get_coordinates(
-        matrix.velocity_neighbor
-    ) / np.linalg.norm(
+    delta_v = (
         shapely.get_coordinates(matrix.velocity)
         - shapely.get_coordinates(matrix.velocity_neighbor)
     )
+    v_rel = delta_v / np.linalg.norm(delta_v, axis=1)[:, np.newaxis]
 
     v_rel_norm = np.linalg.norm(v_rel, axis=1)