use scipy.fft instead of pyfftw

fresnel/vacuum.py

 import numpy as np
-from pyfftw.interfaces.numpy_fft import fftn, ifftn
+from scipy.fft import fftn, ifftn
 from scipy.signal import fftconvolve
 
 from . import hankel
@@ -58,9 +58,11 @@ class FresnelTFPropagator:
 
         self._kernel = fresnelTFKernel(shape, fresnelNumbers)
 
-    def __call__(self, u):
+    def __call__(self, u, workers=-1):
 
-        uprop = ifftn(self._kernel * fftn(u))
+        ft_u = fftn(u, workers=workers)
+        ft_u *= self._kernel
+        uprop = ifftn(ft_u, workers=workers)
 
         return uprop