[MTR4] Rotterdam Metro RSG3

This pack adds the Rotterdam Metro's (R/H)SG3 units to the MTR mod.

The RSG3 is a 3-car electric multiple unit used by the RET on the Rotterdam Metro. They are often coupled in double traction.

Aside from the real RET and Randstadrail liveries, this pack also contains two semi-fictional variants in full RET or full R-net branding instead of shared.