add ability to account for primary body's oblateness in kepler solvers

[mdbernard]

In kepler.py, solve_kepler_chi doesn't account for the oblateness of the primary body. It would be useful to make the following changes:

• Replace parameter mu with a specified CelestialBody instance
• Add with_oblateness parameter, defaulting to False
• If the user specified with_oblateness=True, account for the oblateness of the given primary body