Generalize jacobian to use arbitrary plane

[jhod0]

This is just a tiny change to let the Tracer compute the lensing jacobian for an arbitrary pair of image planes.

The current version only uses the default pair [0, -1], here I let change the signature to let the user specify which planes. But it still defaults to the obvious (first plane, last plane).

Let me know if there's any testing I need to do on this