Modpack refactor (#4)

Split this mod into a set of sub-mods in a modpack, and in the process did a whole bunch of renovations.

* updated Subterrane's API to allow for more patterned placement of things
* added "warrens"
* clean separation of flooded and non-flooded caverns
* rearranged biomes to make cavern layers more distinct
* added oil layer
* added underworld layer

df_mapitems/veinstone.lua

@@ -0,0 +1,16 @@
+-- internationalization boilerplate
+local MP = minetest.get_modpath(minetest.get_current_modname())
+local S, NS = dofile(MP.."/intllib.lua")
+
+minetest.register_node("df_mapitems:veinstone", {
+	description = S("Veinstone"),
+	_doc_items_longdesc = df_mapitems.doc.veinstone_desc,
+	_doc_items_usagehelp = df_mapitems.doc.veinstone_usage,
+	tiles = {"default_stone.png^dfcaverns_veins.png"},
+	groups = {cracky = 3, stone = 1, lava_heatable = 1},
+	_magma_conduits_heats_to = "default:cobble",
+	is_ground_content = true,
+	light_source = 2,
+	drop = 'default:cobble',
+	sounds = default.node_sound_stone_defaults(),
+})