Fuse Resample and Sample classes

To facilitate the current refactoring and future extensions of our sample model hierarchy, I propose to fuse Resample and Sample classes.

Thereby we avoid the duplication of information in pairs of classes (ReX vs X) and the duplication of entire class hierarchies (Struct vs IReStruct, IParticle vs IReParticle).

The current solution is meant to enforce clean of out-of-place refinement. This idea is not fully realized anyhow: One of the first refinement steps, the expansion of LayerStack, is actually done in-place.