enh: "densify" voxel-wise nonlinear mappings with interpolation

Interpolate coordinates when the coordinates to be mapped fall off-grid of the field data array.

Closes: #155.

Author: oesteban

nitransforms/nonlinear.py

@@ -21,6 +21,7 @@
     SpatialReference,
     _as_homogeneous,
 )
+from scipy.ndimage import map_coordinates
 
 
 class DenseFieldTransform(TransformBase):
@@ -145,10 +146,19 @@ def map(self, x, inverse=False):
             raise NotImplementedError
         ijk = self.reference.index(x)
         indexes = np.round(ijk).astype("int")
-        if np.any(np.abs(ijk - indexes) > 0.05):
-            warnings.warn("Some coordinates are off-grid of the field.")
-        indexes = tuple(tuple(i) for i in indexes.T)
-        return self._field[indexes]
+
+        if np.all(np.abs(ijk - indexes) < 1e-3):
+            indexes = tuple(tuple(i) for i in indexes.T)
+            return self._field[indexes]
+
+        return map_coordinates(
+            self._field,
+            ijk,
+            order=3,
+            mode="constant",
+            cval=np.zeros(self.reference.ndim),
+            prefilter=True,
+        )
 
     def __matmul__(self, b):
         """