Improve robustness to crop growing seasons longer than 365 days

I found an old task in my to-do list that I've unfortunately lost the context for:

> Where all does code assume growing seasons < 365 days? Is it just in `idpp` calculation?

This is only possible with `mxmat = .false.` for now, which is not something people should normally run with. Even then, I'm not sure there aren't guardrails to force a harvest at day 364. I'm thus marking this as low-priority. However, it might have implications for the fruit tree PR (#1966).