specify maximum possible latency in `step_adjust latency`

I was running into some cases near the start of a season when some geos had less data than others, and thus wouldn't be eligible for a forecast. One way to solve this is in https://github.com/cmu-delphi/epipredict/pull/408, and lets you exclude specific geos from consideration. A different way of addressing this would be to simply disregard any latency over `max_latency`, say 4 weeks for the above example. In this case, you'd probably want to warn if no points remain after throttling the latency.