TerrainGen: Speed optimizations, typo fixes
Replaced usage of "skip" boolean with elif's. Deleted unused self.skip_over
Corrected "plain" -> "plains" as its spelled in globals
Corrected if a == b or c: -> if a == b or a == c:
Localized self.rand.choice
TerrainGen: Fixed plants preventing nearby sector generation
Using a dict instead of a tuple as "x in dict" is faster