Additional Vegetation Conditions (Ex: Subsurface)

[DeniedExistence]

Hey, I've played around a little with the new 'Alien life' history option in 1.04b6, and I'm rather appreciating it. However, my main annoyance is it always generates forests/cloud forests when used on inhospitable planets. While this is ok, I think a superior solution would be to add a few additional Vegetation Levels:

- Sub-surface vegetation; plant life not present on open surface, but within protected caverns. Could influence the presence of worm-like xenofauna. (Think: Dune)
- Airborne vegetation; plant life spends full life-cycle off the ground. Very high xenobiological hazard.
- Migratory vegetation; plant life has taken on ambulatory, multi-cellular patterns. (Could go very far with this one, such as having forests that slowly migrate between hexes)