Rstan -- maintenance / future prospects?

[jonah]

@hsbadr

May I create a new branch experimental and maintain it (and target it in the current PRs)? This will allow us to consolidate the recent development and then we can merge it into develop when we all agree that it’s ready.

Sounds good to me! Maybe @bgoodri or @rok_cesnovar will have a reason not to do that, but I can’t think of one.