Increase speed of Delta encoding using np.subtract (#584)

* Use inplace subtract in Delta encoding

* ruff reformatting

* Add a comment explaining use of `subtract` for speed

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Co-authored-by: David Stansby <[email protected]>

Author: ehgus

numcodecs/delta.py

@@ -63,7 +63,8 @@ def encode(self, buf):
         enc[0] = arr[0]
 
         # compute differences
-        enc[1:] = np.diff(arr)
+        # using np.subtract for in-place operations
+        np.subtract(arr[1:], arr[0:-1], out=enc[1:])
 
         return enc