simplify slicing of particles, and add support for ParticleComposition

This takes currently place in file Slicer.cpp, fct Compute::Slicing::createParticleInSlice. For a ParticleCoreShell, we get a ReCoreShell; for a ParticleComposition, nothing. This can be improved by returning not just one IReParticle, but as many as needed. No more need for ReCoreShell, and easy support for ParticleComposition.

At this occasion, also consider how materials can be averaged at once, so that we don't need slice2admixtures etc.

Update 15sep: Much has been done in branches i335-i335e. Still todo:

• Sliced version of example CoreShellNanoparticles|2, to validate slicing of compounds and core-shell particles.
• other ways of validation?
• understand (and simplify?) the z scales in ReSample.cpp (functions makeReLayout, SliceIndexSpan, zToSliceIndex)
• get rid of slices2admixtures? (resolved in !1057 (merged))