Remove ud.LoadManager for LoadOrNewManager

In the UD package, there is a Manager object which can be created by both a NewManager and a LoadManager call. Load was called if New returned a specific error to the caller. This MR introduces LoadOrNewManager which simply combines these two functions, with Load's logic moving to where New would have returned the previously mentioned error. This allows the caller to simple call LoadOrNew. There are not bindings for UD at the moment, so this should ease future implementation.