feat(advanced hardening relations): Add parameterized hardening relations for stellar scattering

@cjharris-astro has been working on parameterized stellar scattering relations for a project. I would like these merged into holodeck's dev branch. Their dynamical friction stuff should probably also be merged in, but I can make a separate issue for that. This can wait until after CJ's papers have been submitted / posted to arXiv. 