Enable generating objects on sides or bottom of nodes at mapgen time (#2)

* biome_lib.register_on_generate now supports spawn_on_sides and spawn_on_bottom flags
* biome_lib.register_on_generate now passes node position and face direction to node placement function
* Replacing spaces with tabs
* Fixing API to ensure it behaves same way it used to, when not passing spawn_on_sides or spawn_on_bottom flags.

api.lua

@@ -309,12 +309,34 @@ function biome_lib.populate_surfaces(b, nodes_or_function_or_model, snodes, chec
 		local spawned = false
 		while tries < 2 and not spawned do
 			local pos = in_biome_nodes[math.random(1, num_in_biome_nodes)]
-			if biome.spawn_replace_node then
+
+			local will_place = true
+			local fdir = nil
+			if biome.random_facedir then
+				fdir = math.random(biome.random_facedir[1], biome.random_facedir[2])
+			end
+
+			if biome.spawn_on_side then
+				local onside = biome_lib.find_open_side(pos)
+				if onside then
+					pos = onside.newpos
+					fdir = onside.facedir
+				else
+					will_place = false
+				end
+			elseif biome.spawn_on_bottom then
+				if minetest.get_node({x=pos.x, y=pos.y-1, z=pos.z}).name == "air" then
+					pos.y = pos.y - 1
+				else
+					will_place = false
+				end
+			elseif biome.spawn_replace_node then
 				pos.y = pos.y-1
 			end
+			
 			local p_top = { x = pos.x, y = pos.y + 1, z = pos.z }
 
-			if not (biome.avoid_nodes and biome.avoid_radius
+			if will_place and not (biome.avoid_nodes and biome.avoid_radius
 					and minetest.find_node_near(p_top, biome.avoid_radius
 					+ math.random(-1.5,2), biome.avoid_nodes)) then
 				if biome.delete_above then
@@ -346,10 +368,6 @@ function biome_lib.populate_surfaces(b, nodes_or_function_or_model, snodes, chec
 						biome_lib.dbg("An L-tree was spawned at "..minetest.pos_to_string(p_top), 4)
 						spawned = true
 					else
-						local fdir = nil
-						if biome.random_facedir then
-							fdir = math.random(biome.random_facedir[1], biome.random_facedir[2])
-						end
 						local n=nodes_or_function_or_model[math.random(#nodes_or_function_or_model)]
 						minetest.swap_node(p_top, { name = n, param2 = fdir })
 						biome_lib.dbg("Node \""..n.."\" was randomly picked from a list and placed at "..minetest.pos_to_string(p_top), 4)
@@ -357,15 +375,11 @@ function biome_lib.populate_surfaces(b, nodes_or_function_or_model, snodes, chec
 					end
 				elseif objtype == "string" and
 				  minetest.registered_nodes[nodes_or_function_or_model] then
-					local fdir = nil
-					if biome.random_facedir then
-						fdir = math.random(biome.random_facedir[1], biome.random_facedir[2])
-					end
 					minetest.swap_node(p_top, { name = nodes_or_function_or_model, param2 = fdir })
 					biome_lib.dbg("Node \""..nodes_or_function_or_model.."\" was placed at "..minetest.pos_to_string(p_top), 4)
 					spawned = true
 				elseif objtype == "function" then
-					nodes_or_function_or_model(pos)
+					nodes_or_function_or_model(pos, fdir)
 					biome_lib.dbg("A function was run on surface node at "..minetest.pos_to_string(pos), 4)
 					spawned = true
 				elseif objtype == "string" and pcall(loadstring(("return %s(...)"):