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 +21,8 @@
     SpatialReference,
     _as_homogeneous,
 )
+from scipy.ndimage import map_coordinates
+
 
 
 class DenseFieldTransform(TransformBase):
@@ -134,8 +136,15 @@ 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 displacements field.")
+        if np.any(np.abs(ijk - indexes) > 1e-3):
+            return map_coordinates(
+                self._field,
+                ijk,
+                order=3,
+                mode="constant",
+                cval=np.zeros(self.reference.ndim),
+                prefilter=True,
+            )
         indexes = tuple(tuple(i) for i in indexes.T)
         return self._field[indexes]