* bug fixes
Fixes wrong new_y_min when shift_existing_biomes() shifts a biome below the nether floor,
find_surface_anchorPos() no longer assumes y=0 will be outside the nether.
Nil-reference fixed when a mods tries to register a portal after mods are finished loading, but the portal shape+material was already registered by another mod.
* Make ShiftExistingBiomes available via API
makes the ShiftExistingBiomes function available to other mods via the nether global, since it's not a simple function and biomes would also need to be shifted if another mod wants to add a second nether layer.
* Allow layers to extend the depth of nether effects
Mods can set/lower nether.DEPTH_FLOOR_LAYERS when creating a layer under the nether. This allows multiple layer mods to know where their ceiling should start, and to be included in the effects which only happen in the nether.
* document nether API
More of a tentative interop guide than an API.
Use snake_case for API functions.
