Restrict Nether Portal Generation to above Y level 35

Especially if you build a portal deep in a cave where you can find lava pools, you often end up with a portal in a cave that is below the lava ocean. From there it is almost impossible to get to the surface, because any attempt to mine up floods your tunnel with lava. Nether portals should not be able to generate below y level 35 - (the level of the lava oceans).