add target to DoInteraction interface
I do not like the current interface of DoInteraction.
InteractionProcess::DoInteraction(vProjectile) is unphysical, as interactions necessarily involve two systems.
Is it possible to move the call to the environment to ask for the target material outside DoInteraction? (I am a little bit fuzzy on whether 'outside' is the ProcessSequence or Cascade::Step.)
Then DoInteraction would have to be changed to
For Cascade we would not need a particle stack (particles with energy and momenta) as target argument, particle codes would be enough. However for the to-be-defined HepMC/CRMC mode, it helps to be able to pass arbitrary reference frames. Otherwise we are boosting back and forth unnecessarily (like in the current CRMC). Another point is that currently the frame-defining-code (target momentum is set to zero) is located in the interface between corsika 8 and the interaction models.
Related to #56 (closed), eg. not urgent but may be more complicated to implement in the future.