Revert change to delta filter (#677)

Author: dstansby

numcodecs/delta.py

@@ -63,12 +63,7 @@ def encode(self, buf):
         enc[0] = arr[0]
 
         # compute differences
-        # using np.subtract for in-place operations
-        if arr.dtype == bool:
-            np.not_equal(arr[1:], arr[:-1], out=enc[1:])
-        else:
-            np.subtract(arr[1:], arr[:-1], out=enc[1:])
-
+        enc[1:] = np.diff(arr)
         return enc
 
     def decode(self, buf, out=None):