Missing pieces for case studies

Here is what we miss to match the [*exoplanet* case studies](https://gallery.exoplanet.codes/):
- [ ] The [Van Eylen et al. (2019)](https://arxiv.org/abs/1807.00549) eccentricity distribution

PRs are welcome!

Start by comparing the linked exoplanet case studies with the Van Eylen et al. (2019) paper to identify the intended eccentricity distribution and its required behavior. Then locate the corresponding case-study modeling entry point in jaxoplanet; the work is done when that distribution is supported and the case study matches the reference.

Written by the indexing model from the issue text.