added circular Kerr periastron advance

[x-m-p]

add self-force correction to the circular limit of the periastron advance for Kerr from https://arxiv.org/abs/1907.11083

[nielsw2]

Thanks for submitting this PN series. Looking at the paper it looks like the series is an expansion in q (the spin of the primary), and valid through q^2. We should have a way to express this in the series expansion. Either each term has a +O(q^3) or the entire series needs such a term.

@Chriskavn @barrywardell any ideas on the best way to handle this? Should each PN order be it's own expansion in q, or should we do something like + O(q^3) for the entire series?

[barrywardell]

I think each term should probably be a series in q. It's possible that we could obtain an exact-in-q expression at some time in the future as has been done for other quantities.