Improve distance check.

We'll check the horizontal distance between the character's and the
plant's edges and compare that to the normal reach of a human. The size
of the plant should not make any difference; the only thing of
importance is how far the actor can reach.
Only harvest adult plants.

If there's a "sizeAdult" property available; else just try to harvest
them as they are.
Added function for measuing entity edge distance.

This is useful when trying to determine in tasks whether an actor is
close enough to another object to be able to perform a task.
Just measuring from the base won't do, as then we run the risk of too
thick entities not being possible to perform tasks with.
Correct documentation.
Alter how proximity is calculated.

We now try to see if the entity performing the task can reach the
entity which is acted upon. This is independent of the size of the
entity; the only thing that's of interest is the distance between the
edges of the two entities and the reach of a standard human.