Hybrid Discrete/Continuous

[teubert]

Implement way of handling hybrid discrete/continuous models (where some states are continuous and others are discrete) that would support integration methods on discrete part.

For now we're requiring users to do this manually in their implementation. If we continue with that solution we should provide an example (e.g., they implement dx on the discrete parts and use utils.next_state functions in their next_state, adding on the discrete state afterwards).

Idea from:
"I was browsing the centrifugal pump model and it looks like it is considered a discrete model because of the QLeak state but all the other hidden states are continuous. I'm curious if you have any thoughts on how these integration methods could be utilized in a hybrid model, or if it should just be left up to the person writing their model including the integration method in their next state function."

Originally posted by @MikeAndSpencer in #529 (review)