Biomes: Add 'get heat', 'get humidity', 'get biome data' APIs

'get biome data' returns biome id, heat and humidity.
Clean up nearby lines in lua_api.txt.

src/script/lua_api/l_mapgen.h

@@ -25,9 +25,21 @@ class ModApiMapgen : public ModApiBase
 {
 private:
 	// get_biome_id(biomename)
-	// returns the biome id used in biomemap
+	// returns the biome id as used in biomemap and returned by 'get_biome_data()'
 	static int l_get_biome_id(lua_State *L);
 
+	// get_heat(pos)
+	// returns the heat at the position
+	static int l_get_heat(lua_State *L);
+
+	// get_humidity(pos)
+	// returns the humidity at the position
+	static int l_get_humidity(lua_State *L);
+
+	// get_biome_data(pos)
+	// returns a table containing the biome id, heat and humidity at the position
+	static int l_get_biome_data(lua_State *L);
+
 	// get_mapgen_object(objectname)
 	// returns the requested object used during map generation
 	static int l_get_mapgen_object(lua_State *L);