Implement periodic re-optimization of hour angles

[dkirkby]

The survey simulations currently use the same design HAs during the whole survey, but the underlying Optimizer class is designed to support periodic re-optimization, e.g., to adjust to better / worse weather than initially assumed.

This issue is to update desisurvey.plan.Planner.afternoon_plan to periodically perform this re-optimization (during the annual monsoon shutdown?) and run some simulations to study the impact.