chg: scaling: Add filtered bandwidth function

to calculate the filtered bandwidth for each relay.

sbws/lib/scaling.py

+from statistics import mean
+
+
+def bw_measurements_from_results(results):
+    return [
+        dl['amount'] / dl['duration']
+        for r in results for dl in r.downloads
+    ]
+
+
+def bw_filt(bw_measurements):
+    """Filtered bandwidth for a relay.
+
+    It is the equivalent to Torflow's ``filt_sbw``.
+    ``mu`` in this function is the equivalent to Torflow's ``sbw``.
+    """
+    mu = mean(bw_measurements)
+    bws_gte_mean = filter(lambda bw: bw >= mu, bw_measurements)
+    return mean(bws_gte_mean)