Add windowing to paradiag.solve()

[JHopeCollins]

paradiag should take an argument for the number of windows to solve for. This will need:

• an extra argument to init
• a loop in the solve() method to move to the next_window() and run the nonlinear solve
• Possibly adding a callback argument to solve() to call between each window. This means we could save snapshots, calculate CFL etc on the fly rather than having to store a big time series list to do these steps at the end.

[colinjcotter]

I would prefer to have the number of windows as an argument of solve.

[JHopeCollins]

Yes, that makes much more sense!