Charge carrier reflection on surface

The surface reflection introduced in MR !1014 (closed) is modified as follows:

• the electric field and doping concentration for the diffusion modeling are taken from the position before the drift modelling (last_position) as also addressed in !1035 (merged). This avoids that the diffusion uses a null field / concentration, if the charge carrier drifts outside of the sensor
• the charge collection and surface reflection are carried out before other effects, such as trapping, recombination, etc. are modeled. Otherwise nonphysical effects are observed
• the surface effect is modified to actually reflect the charge carrier at the surface (note the z-position change). This is necessary to avoid nonphysical behavior, if e.g. the electric field at the surface is zero. The current implementation may lead to charges "bouncing" along the surface (which is also somewhat nonphysical). Yet, this is observed to model the measurement well
• currently the reflection is only implemented for the z-boundary of the sensor. Other use-cases seem unlikely, yet we can discuss this

Finally, the author-list has been updated, adding @cscharf and me.

Tagging @cscharf