Fix l2 parameter in SAL verification (#324)

Author: EsmailGhaemi

pysteps/verification/salscores.py

@@ -300,12 +300,9 @@ def _sal_l2_param(prediction, observation, thr_factor, thr_quantile, tstorm_kwar
     maximum_distance = sqrt(
         ((observation.shape[0]) ** 2) + ((observation.shape[1]) ** 2)
     )
-    obs_r = (
-        _sal_weighted_distance(observation, thr_factor, thr_quantile, tstorm_kwargs)
-    ) * (np.nanmean(observation))
-    forc_r = (
-        _sal_weighted_distance(prediction, thr_factor, thr_quantile, tstorm_kwargs)
-    ) * (np.nanmean(prediction))
+    obs_r = _sal_weighted_distance(observation, thr_factor, thr_quantile, tstorm_kwargs)
+    forc_r = _sal_weighted_distance(prediction, thr_factor, thr_quantile, tstorm_kwargs)
+
     location_2 = 2 * ((abs(obs_r - forc_r)) / maximum_distance)
     return float(location_2)