Add check that checkpoints and diagnostics output are synced up

[Sbozzolo]

As we move towards long simulations, checkpoints become more and more important. While adding support for diagnostics that work across restarts is possible, that's something that is not easy to implement (without imposing various restrictions).

As a first step, it would be good to at least ensure that diagnostics and checkpoitns are output at the same schedule (e.g., every month)