Farming Update, take 3

Add an attribute "grow" to seeds to allow variable plant growth times.

Tested and it compiles. I'm really hoping to get this merged this time, and I'll do whatever needs to be done to fix it.
Nitpicking: the `calendar::turn >= seed.bday + plantEpoch * 2` part can be removed. If that part was true, the first branch above ("f_plant_seedling") would already be used and the other branches would be ignored.