Add more intuitive way of defining spread duration when running deterministically

[jordan-evens]

Right now, deterministic mode doesn't have anything special in it aside from the chance of spreading being 100% regardless of ROS. This means that it should still be checking the minimum day/night FFMC thresholds and the sunrise/sunset offsets to determine if a fire should be spreading.

If only the --deterministic flag is used, this means a fire will generally spread at full FBP values for the full duration of daylight, unless hourly FFMC exceeds the threshold so that it continues overnight. This may not be obvious, since there aren't hard spread thresholds like there are with other models (e.g. ISI > W, FWI > X, WS > Y, RH < Z)