Have the flat tank-end be the default 'none' option for the tanks (as a distinct model), and alternatively offer a tank-top/bottom/mount option similar to the top on the MUS parts (bare domed tank).

This change will remove ~1k tris from rendering on the tanks when other mounts are used, while effecting no other real downside.

Possible compat problems may arise where old vessels using the 'none' mount type will end up with no model/etc (as the new 'tanks-none' mount will be a distinct mount variety, can catch cases on-load of the mount-def not being found, and reverting to default -- the new 'tanks-none').