Add a windowing loop with callback to `paradiag.solve()`

[JHopeCollins]

Also adding some utility functions such as:

• paradiag.get_timestep()
• paradiag.set_timestep()
• paradiag.for_each_timestep()

[JHopeCollins]

As well as the windowing loop in paradiag.solve(), I also added a few QoL functions that I needed for the for_each_timestep() method. A summary of the changes:

1. paradiag.solve() now takes the number of windows to solve, as well as two callbacks: one called just before the window solve (so w_all is the initial conditions), and one called just after (so w_all is the solution for the window)
2. paradiag.for_each_timestep(): calls a callback with the solution at each timestep in turn. Useful for postprocessing.
3. paradiag.set/get_timestep(): set and extract solution at individual timesteps from all-at-once function.
4. paradiag.shift_index(): logic for shifting between window/slice indices and dealing with negative indices
5. paradiag.check_index(): check a timestep index is within stated range.
6. Using these methods to simplify some bits of the code.