Added Nether Gateway Chance Correction

Modified RiftGenerator to correct for the rarity of Rift Gateways in the
Nether. Our config settings allow us to set the probability that we will
attempt to generate a gateway in a given chunk. However, that doesn't
guarantee that a gateway will generate.

I collected a lot of data and determined that generation succeeds in the
Nether only about 15% of the time. That's compared to 30% in the
Overworld when counting oceans (which always fail) and about 75%
(sometimes higher) when traveling mainly on land.

RiftGenerator now corrects for this by multiplying the chance of
attempting to generate gateways in the Nether by 4. Gateways in the
Nether are still relatively rare and hard to find, but you'll
occasionally come across them now. Also reorganized the code a little
for clarify.