Add driver/callback docs

[MTCam]

fixes #427

[tulioricci]

Trim this a bit to align with the text

[tulioricci]

I was wondering if we should add a section about the RHS too, particularly because of make_operator_fluid_states (i.e., we gotta pass the limiter). These two should cover the most important pitfalls concerning limiters.

[MTCam]

I agree that we need better documentation and developer guidance for drivers. It would be my preference to document this and your suggestion in a sort of detailed driver walk-through. For now, maybe it would suffice to something like this:

  Consider these snippets from the *RK4* time integrator, and example *rhs* function:

   | def rk4_step(state, t, dt, rhs):
   |     """Take one step using the fourth-order Classical Runge-Kutta method."""
   |     k1 = rhs(t, state)
   |     k2 = rhs(t+dt/2, state + dt/2*k1)
   |     k3 = rhs(t+dt/2, state + dt/2*k2)
   |     k4 = rhs(t+dt, state + dt*k3)
   |
   |     return state + dt/6*(k1 + 2*k2 + 2*k3 + k4)
   
   | def rhs(t, state):
   |     """Return the RHS of the conservation system."""
   |     filtered_state = apply_filter(state)
   |     return rhs_operator(t, filtered_state)

What I really want in the end is documentation that covers an integrated driver with all of its components and constructs. Would be nice if those docs could be (largely) generated from the specific examples and their inline documentation, but I think we need to restructure the example drivers somewhat in order to easily wedge their docs into the automatic online documentation.

[tulioricci]

Capisce. There are a few PRs concerning driver updates, so we can put more doc details in those then.

[tulioricci]

👍