Restructure elementGeometryType

[MarAlder]

I propose to adopt the predefined elements:

• add scaling to the pre-defined primitive geometries, if you need ellipsoids, skewed frustums, etc.
• alternatively to scaling we could replace sphere by ellipsoid
• genericGeometryComponent -> renamed to externalGeometry. I don't like the previous name adopted from older CPACS releases. The term component conflicts with the term element which we want to use for the library elements sections.
• add genericGeometry (see Generic geometry type #844 for further details)

Referencing in aircraft/model should then only allow for translation and rotation, but not scaling:

This supports the unambiguous interpretation of the center of gravity (otherwise it is not entirely clear what happens when scaling to the plane of flight for a given primitive body).