Break too long lines and simplify the code

- Add boost_cart:get_rail_groups to get the default rail groups
- Remove duplicated code
- Do not boost the cart backwards when the brake rail gets activated
- Attempt to fix the cart swing problem
- Trim tailings and tidy up code
- TODO: Document all the stuff

functions.lua

@@ -213,3 +213,14 @@ function boost_cart:register_rail(name, def)
 
 	minetest.register_node(name, def)
 end
+
+function boost_cart:get_rail_groups(additional_groups)
+	-- Get the default rail groups and add more when a table is given
+	local groups = {dig_immediate = 2, attached_node = 1, rail = 1, connect_to_raillike = 1}
+	if type(additional_groups) == "table" then
+		for k, v in pairs(additional_groups) do
+			groups[k] = v
+		end
+	end
+	return groups
+end