Replace inner kernel for pickling

Motivation: When I want to enable pickling of symbolic expression in Pytential (pytential!133 (merged)), replace_inner_kernel is undefined at this line.

I think the intention here is to move the derivative inside the inner kernel, but I'm not sure whether my implementation is sensible or correct.

cc @inducer

To-do: Review !133 (merged) when this is in.

sumpy/kernel.py

@@ -797,6 +797,9 @@ class AxisTargetDerivative(DerivativeBase):
         expr = self.inner_kernel.postprocess_at_target(expr, bvec)
         return expr.diff(bvec[self.axis])
 
+    def replace_inner_kernel(self, new_inner_kernel):
+        return type(self)(self.axis, new_inner_kernel)
+
     mapper_method = "map_axis_target_derivative"