DPRKN6 interpolants after events need `add_steps!`

After an event, the add_steps! function is called for re-building the interpolant cache. This is separate from perform_step! because lots of algorithms have different needs if just building the interpolant, and in many cases this can be sped up. And this is only required for special interpolants because there's a Hermite fallback which just calculates the derivatives on the two ends.

However, noticeably missing are the SSPRK and Rodas4 methods. We should get those fixed up.