Notebook showing an example of using reboundx in exoplanet

**Is your feature request related to a problem? Please describe.**
I am interested in computing the jacobian of the state of a system
with respect to the initial conditions using reboundx. For example,
I would like to integrate the outer four giant planets of the solar
system. I'm planning to compare this with the NbodyGradient as
a cross-comparison of both.

**Describe the solution you'd like**
A Jupyter notebook or script showing an example of how to set
up and run a system in exoplanet using reboundx (fine to just
use the code which was used in implementing/debugging the
incorporation of reboundx into exoplanet).

**Describe alternatives you've considered**
I could just run reboundx from rebound directly, but I'm also
interested in learning how to do this from within exoplanet to,
for example, run photodynamic models.

**Additional context**
None at the moment!

Start by locating the existing code used to implement or debug reboundx incorporation into exoplanet, then review the reboundx and NbodyGradient usage mentioned in the issue. Add a Jupyter notebook or script that sets up and runs the requested exoplanet example, with the jacobian workflow demonstrated and enough output to compare approaches.

Written by the indexing model from the issue text.