Dont force interpolation fill value to nan, allow extrapolation

nabu/preproc/distortion.py

@@ -74,7 +74,6 @@ class DistortionCorrection:
         check_supported(correction_method, self.correction_methods.keys(), "correction method")
         self.corrector = self.correction_methods[correction_method]
         self._corrector_kwargs = correction_kwargs or {}
-        self._corrector_kwargs["fill_value"] = np.nan
 
 
     def estimate_distortion(self, image, reference_image):
@@ -84,8 +83,10 @@ class DistortionCorrection:
 
     def correct_distortion(self, image, coords):
         image_corrected =  self.corrector(image, coords, **self._corrector_kwargs)
-        mask = np.isnan(image_corrected)
-        image_corrected[mask] = image[mask]
+        fill_value = self._corrector_kwargs.get("fill_value", None)
+        if fill_value is not None and np.isnan(fill_value):
+            mask = np.isnan(image_corrected)
+            image_corrected[mask] = image[mask]
         return image_corrected
 
     correct = correct_distortion

nabu/preproc/tests/test_ctf.py

@@ -125,7 +125,7 @@ class TestCtf:
                 "correction_spike_threshold": 3.0
             },
             correction_method="interpn",
-            correction_kwargs=None
+            correction_kwargs={"fill_value": None}
         )
         flats = FlatFieldArrays(
             [1200] + list(self.img_shape_vh),