Audit generator settings and:

• Remove settings that will never be used. (eg seaLevel)
• Assess and adjust defaults where necessary. (eg useRavines = false)
• Implement currently unimplemented settings.
• Remove all config settings that an equivelent generator setting replaces.

The remaining items in this issue are still worth doing, but can probably wait until we release for 1.12

In the GenLayer system I am making seaLevel is completely hardcoded, so that setting won't exist soon.