Support bi-, tri-, quad- -couplers and - adapters

[aandergr]

These give the option to use more than one stack-mounted engine, and offers different choices of where to put fuel tanks.

[popiel]

This also has interesting interactions with:

• SFB staging
• possible radial geometries
• small to large radius converters (e.g. bi-coupler to c7 slanted works to get side-by-side large)