Revisit wrapped solvers

[patrick-kidger]

These don't really work properly at the moment. For example

class MySolver(AbstractWrappedSolver):
    ...

diffeqsolve(solver=MySolver(Kvaerno3()), stepsize_controller=PIDController(...), ...)

does not work because MySolver doesn't have a nonlinear_solver attribute for the PIDController to apply its rtol and atol to.