Add return type annotation

HearingTest.py

@@ -127,7 +127,7 @@ class HearingTest:
         return text
 
 
-def mean_two_arrays(a: np.ndarray, b: np.ndarray):
+def mean_two_arrays(a: np.ndarray, b: np.ndarray) -> np.ndarray:
     return np.mean(np.array([a, b]), axis=0)