Add a "vitality" property

A "vitality" property should be added, which determines how things grow.

Currently the plant code works in such a way that any tick where there's no nourishment (like with an uprooted plant) the mass of the plant is decreased.
This isn't really correct though; any plant that's withering will contain most of it's mass, but will instead lose vitality (though some mass is initially lost due to dehydration; we'll ignore that). When the vitality reaches zero the plant is dead (but still exists in the world).
The "vitality" property should affect how much mass the plant will receive from nourishment operation. It will be normalized and decrease when the plant withers, and increase when it's nourished.
It's separate from status. Status determines the overall physical shape of the plant; when it's zero the plant is destroyed. But "vitality" determines how much "alive" the plant is.