[geom] Fix Graph.shifted/at

phi/geom/_graph.py

@@ -137,7 +137,14 @@ def bounding_half_extent(self) -> Tensor:
         return self._nodes.bounding_half_extent()
 
     def at(self, center: Tensor) -> 'Geometry':
-        return Graph(self.nodes.at(center), self._edges, self._boundary)
+        raise NotImplementedError("Changing the node positions of a Graph is not supported as it would invalidate distances.")
+        # warnings.warn("Changing the node positions of a graph triggers re-evaluation of distances.", RuntimeWarning, stacklevel=2)
+        # return Graph(self.nodes.at(center), self._edges, self._boundary, bounding_distance=self._bounding_distance is not None)
+
+    def shifted(self, delta: Tensor) -> 'Geometry':
+        if non_batch(delta).non_channel.only(self._nodes.shape):  # shift varies between
+            raise NotImplementedError("Shifting the node positions of a Graph is not supported as it would invalidate distances.")
+        return Graph(self.nodes.shifted(delta), self._edges, self._boundary, deltas=self._deltas, distances=self._distances, bounding_distance=self._bounding_distance is not None)
 
     def rotated(self, angle: Union[float, Tensor]) -> 'Geometry':
         raise NotImplementedError