Add an example of how to use `gw_eccentricity` with [`TEOBResumS`](https://bitbucket.org/eob_ihes/teobresums/src/e11ebe3d6d96ac0bba50a72fa8a56f7f16d77e52/Python/Examples/ExampleEccentricBBH.py?at=dev%2FDALI)
