make double_flatfield more readable with sigma_filter

nabu/preproc/double_flatfield.py

@@ -115,6 +115,8 @@ class DoubleFlatField(CCDProcessing):
         self.output_is_mlog = output_is_mlog
         self.average_is_on_log = average_is_on_log
         self.sigma_filter = sigma_filter
+        if self.sigma_filter is not None and abs(float(self.sigma_filter)) < 1e-4:
+            self.sigma_filter = None
         self.filter_mode = filter_mode
         proc = lambda x,o: np.copyto(o, x)
         if self.input_is_mlog:
@@ -158,7 +160,7 @@ class DoubleFlatField(CCDProcessing):
 
         acc /= radios.shape[0]
 
-        if self.sigma_filter is not None and abs(float(self.sigma_filter)) > 1e-4:
+        if self.sigma_filter is not None:
             acc = acc - gaussian_filter(acc, self.sigma_filter, mode=self.filter_mode)
         self.doubleflatfield = self.postproc(acc)
         # Handle small values to avoid issues when dividing