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.
>why 91?

Yeah that's not going to work.  If you want to use an arbitrary number, it's 13 or 14, the default season length.  (A default DDA day maps to roughly 6 wall-calendar days.)  91 is close to reality, which makes the research easier, but then you need scaling to the default.