Previous times were chosen using statistical maths, but reports suggested
this was too long.
I tested by timing an ABM acting on 100 nodes, with interval and chance equal
to the old sapling ABM.
50 at 4m59s.
99 at 24m58s.
100 at 26m58s.
So choose a grow time between 5 and 25 min for tree and bush saplings.
If 'can grow' is false at grow time the timer is reset to 5 min.
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