Extend interpolation functions to integrator/callback

The current interpolation definition only allows you to interpolate over the final value of a solution. I would like to be able to include an interpolation in a PostprocessCallback so I can see how pressure at a point (for example) evolves over time.
From what I can tell from a glance at the code, it wouldn't be a small change. Since the system data you get during a PostProcessCallback is isolated to a single system, I don't think its possible to do an interpolation.

I could see a simple fix using the custom_quantities interface, which provides u_ode and v_ode, and then if these versions of the interpolation functions were made public, it could be accessed there

From what I can tell, the custom_quantities code is the only public description of u_ode and v_ode. I wasn't sure if you consider using u_ode, v_ode = sol.u[end].x to be valid API (for breaking changes purposes)

Start with src/general/interpolation.jl, especially the interpolation functions linked in the issue, and trace how PostprocessCallback exposes system data. Read the custom_quantities interface and its u_ode and v_ode values before deciding how the public API should work. Done means a callback can use interpolation to observe a quantity such as pressure over time without relying on an undocumented access pattern.