Elevation factor.

Lower all plant densities 25% every 16 blocks (it's actually
a fraction each block). This mimics the elevation factor that
exists in reality.
Don't plant saplings under a canopy.

This is to reduce the chance of planting useless saplings,
and should be step 1, the easiest check for valid sapling
location.

Although it's possible for plants to grow underneath large
canopies, especially in a jungle, even without saplings
underneath a canopy the end density will be plenty tight.
Lots of v7 final tweaks and fixes.

Major algorithmic fixes for planting saplings.
Introduce a version number for the plants.csv, so we can alert.
Update Readme with some minor text details
Don't plant saplings near other saplings (8 spaces min.).
Properly exit subloops.
Finish up for v7 release.