[Freeorion-programmers] Planetary environment name change

[Zach L. <wha...@gm...>]

I swapped the names "Superb" and "Optimal" in the DD and in the code. 
"Superb" means "great", but "Optimal" means "best", so Optimal should
be what we call a Gaia world, instead of what we call a
homeworld/terran planet.

Zach