add RTW+NEF vehicle combination

[christianzoellner]

as a minimal fix for the requested vehicle sub-types in #1029, I propose the follwing:

The most commonly cited usecase was an NAW-type vehicle (i.e. RTW with extra notarzt), which are needed for transport exercises where it is an important factor in exercise evaluation to be able to differentiate between Notarzt and non-Notarzt transports.

Since from the outside, RTWs and NAWs are not distiguisable, one could add an "RTW+NEF" vehicle type where the image is the two vehicles together. that way, no new images need to be created.
The new vehicle type could be marked as "(transport only)" and not contain any personell, since disembarking a dual vehicle combo and naming the personell might be awkward.

[anonym-HPI]

@christianzoellner NAW was added via this PR #1034, is this issue therefore resolved?