Type of projectile in StackView

The StackView should also return a "normal" StackIterator to the projectile, not only the version that is specific to the "view". The type of the projectile is very relevant in the context of functionality and many of the interface functions. Right now this limits in an artificial way what is possible. E.g. process.GetCrossSection(particle) is no problem, but process.GetCrossSection(projectile) is not possible.