Split underground biome to correspond to surface biomes

In preparation for biome-defined: dungeon materials, cave liquids,
stone type, ores, decorations.
'_ocean' biomes now extend to y = -255 to be deeper than default
mgv5 oceans, and to create 'shallow underground biomes'.
Remove unnecessary biome lists for gravel and silver sand blob ores,
as those are already defined to only appear in default:stone.