Enable multiple scattering in proposal

I would like to re-enable (parts) of the multiple scattering for proposal::ContinuousProcess which has been disabled with commit 2066f920.

This means that the direction of charged particles after a continuous step is changed by proposal.

If you would want to be complete, proposal would also need to change the position of the particle during the continuous step. I tried to implement this on the branch proposal_multiple_scattering_change_particle_direction . However, this is not working without problems: At the moment, corsika::Cascade::step first calculates the distance to the medium transition (taking the magnetic field into account), and applies the continuous steps afterwards. However, if the continuous step changes the particle position, the previous calculation of the distance to the next medium becomes inaccurate (which leads to cases where to particle propagates into the next medium instead of only to the border). From this, I concluded that continuous processes are not supposed to change the particle position. I believe we need to treat multiple scattering together with the geometric tracking. We probably need to discuss this in more detail but it is not of importance for this MR right now.

To estimate the effects of this, I created lateral profiles of EM showers for three different cases:

• No multiple scattering (current master) ("no ms")
• Multiple scattering but only change the particle direction after a continuous step ("ms") (this MR)
• Multiple scattering but change the particle direction and position after a continuous step ("full ms") (branch proposal_multiple_scattering_change_particle_direction )

lateral_distribution.PDF

From this, I concluded that the difference between "no ms" and "ms" is significant, but compared to this, the difference between "ms" and "full ms" can be neglected for now. (Be aware that this plot only includes limited statistics)

Since we want to compare lateral distributions between C8 and C7 for ICRC, I would like to have this on the master if possible (first examinations also showed that with this MR, the lateral profiles of C7 and C8 are in much better agreement now).