Subterranean Planets

Note: I do not plan to develop this mod further. If you're interested in adopting it, let me know! ---- This mod adds three new planet types, which are basically your standard Toxic, Molten, and Frozen worlds, but with subterranean caverns that can support life. These planets cannot be terraformed into normal habitable planets or vice versa, but you can still turn them into Machine Worlds or Ecumenopolises. There are three new civics, one for each planet type, which allow you to start on one; Subterranean origin is a prerequisite. There is also a new "Uninhabitable System" start, which sets your homeworld in a Black Hole, Pulsar, or Neutron Star system. It's meant for subterranean planets, but normal planet classes will work here as well if you desire. Should be compatible with all other mods, including Planetary Diversity and the like.