added Median function

modules/base/pymod/stutil.py

@@ -56,7 +56,7 @@ def Median(xs):
     raise RuntimeError("Can't calculate median of empty sequence")
   sorted_xs=sorted(xs)
   if (len(xs) % 2)==0:
-    return (sorted_xs[len(xs)/2]+sorted_xs[len(xs)/2]+1)/2
+    return (sorted_xs[len(xs)/2]+sorted_xs[len(xs)/2+1])/2
   else:
     return sorted_xs[len(xs)/2]