Need "StackView" object

In order to run modular extra code like thinning on the Stack we need to extend the current infrastructure. For example:

• an interaction produces secondary particles
• only after the collision a thinning algorithm can run
• however, after the collision the particles are already on the main Stack, and e.g. even re-sorted.
• we need an extra "list" or maybe a transparent "StackView' that behaves exactly as a Stack, but only shows the newly created particles. This is a temporary object that is needed only in the end of the Cascade::Step function for a very short time.