Allow volume/shape specialization through non-child classes

I would like to improve our factory mechanism for unplaced volumes "A" in the sense that
the factory should be able to give back specialized volumes (that behave like) B although B is not an A in the C++ class sense.

Details:

So far specialization was only possible for volume types which are related by inheritance.
We could create a FullTube when we asked for a Tube because we have a type FullTube which "is-a" Tube.

Sometimes, the situation is different. For example, a trap might under certain circumstances be a box. However, a box is not a trap (in the C++ class layout) and it would be cumbersome to restructure the class layout.

Idea:

We can solve the problem by automatically creating subtypes of trap that behave like a box.
Hence we look for a mechanism to create `UnplacedImplAs<A,B>` which signify
a type that "is-a" A but that is optimized by using "B".