Use inplace subtract in Delta encoding

Author: ehgus

numcodecs/delta.py

@@ -64,7 +64,7 @@ def encode(self, buf):
         enc[0] = arr[0]
 
         # compute differences
-        enc[1:] = np.diff(arr)
+        np.subtract(arr[1:], arr[0:-1], out = enc[1:])
 
         return enc