Allow a configurable layer level

[pragmatic]

The concept of this generator plus it's companion Giant Caves is great, I have been enjoying exploring some massive caves.

One thing I miss though is that I cannot build structures above ground that are very tall due to only having on average 64 blocks to play with.

I'd like to combine the larger underground components with the default generator's larger overground component. I would suggest giving the option to customise the level to anything within a sensible range (64-192?). I would most likely end up doubling the default world height to 128, rather than tripling as it is currently.