[MTR4] NS DDM / DD-AR / DDZ for MTR

This pack adds the NS DDM, DD-AR and DDZ to the MTR mod.

The DDM is a set of double-decker carriages, meant to be carried behind a locomotive, while the DD-AR and DDZ are multiple units with a motor car, with the DDZ having the modern livery.

They are found in sets of 4 or 6 cars, and often coupled to 8 or 10, though rarely beyond.