Add `slidingRP` optional dependency

[alejoe91]

Suggested by @oliche here: #4682 (comment)

Repo: https://github.com/SteinmetzLab/slidingRefractory

Thoughts? @chrishalcrow @samuelgarcia @zm711 @ecobost @h-mayorquin

[zm711]

I think the one benefit of that is that we have two different interpretations of the Llobet violation calculations. If we pin a dependency then in our provenance docs we can say that the sliding RP is whatever interpretation that Steinmetz repo says it is rather than claiming to have the correct one in our code. On the other hand, for people reading our source code they lose the option to see the current implementation for their analysis. Basically I'm not sure what the right answer is...