Core FormFactors: rotation and slices

Review mechanism for particle rotation and slices.

• Reconsider the API: Are particles centered at center or at bottom (related to multilayer issues)? Where is the rotation center? [Redmine 2471]
• Warn about unsupported situations [Redmine2516].
• Improve over the polyhedral approximation to elliptical shapes [Redmine2299 ?].
• Support rotations of box (and many other FFs?) [Redmine2477].
• Improve test coverage [Redmine2309] #241.
• Review handling of compound particles #335 (closed).
• Ditto for crystals #161 (closed).