Prevent crops from "overwintering"

(This was first described in #1928, which I'm closing as complete, to be replaced with this issue.)

During the crop reparameterization I did for CTSM6.0, one of my main goals was reducing the fraction of harvests where the crop failed to reach a certain level of maturity. I think I succeeded on that front; compare the green vs. blue lines here:

It's tricky, though. The main lever for this is increasing maximum growing season length (PFT parameter `mxmat`), which can have the side effect of warm-season crops in certain regions growing well into the winter. This is obviously unrealistic: In real life, farmers would have harvested before winter settles in, even if the crop didn't reach a useful level of maturity.

It's unclear how much of a problem this is. I've made some figures where I defined "overwintering" as a growing season that spans the new year in the Northern Hemisphere, with the Southern Hemisphere's definition offset by 6 months. This is obviously crude—too-cold temperatures often start much earlier in the year—but it's an easy figure to make with the data I had available. (We can ignore "overwintering" in the tropics and to some extent subtropics.) The only area with an issue seems to be central Asia for cotton (sorry about inconsistency with the years here):

In the GGCMI growing season data, that region is typically harvested around day of year 275-300, so it "shouldn't" be doing this.

If we determine this is actually an issue we want to solve, we could force harvest if no growing degree-days were accumulated in the past, say, 14 days. I'd want to talk with agronomists to better parameterize this.

Winter wheat makes this discussion more complicated—would we want to avoid the possibility of "oversummering" instead? Certainly I would turn off the mechanism proposed above during the vernalization period.