Divide sigma by -2 (#59)

Instead of negating the result of `(sigma ** 2) / 2`, which may be a
Dask Array meaning this would add more nodes to the graph, simply divide
by `-2` instead. This ensures the sign still changes as intended, but
this is included as part of the division step. Thus this avoids
potentially adding more nodes to the Dask graph (assuming `sigma` is not
concrete).

dask_image/ndfourier/__init__.py

@@ -67,7 +67,7 @@ def fourier_gaussian(input, sigma, n=-1, axis=-1):
     )
 
     # Compute Fourier transformed Gaussian
-    scale = - (sigma ** 2) / 2
+    scale = (sigma ** 2) / -2
     gaussian = dask.array.exp(
         dask.array.tensordot(scale, ang_freq_grid ** 2, axes=1)
     )