Determination of the Electric Field at Contacts

The issue came up from #279

Right now, we assign an electric field vector to each grid point of the grid of the electric potential.
The components of the vector is calculated by the differences in potential to the neighboring grid points in the respective dimensions. However, for 2D-contacts, e.g. a plane in `x-y`, a wrong value is determined for the component in `z`: `E_z`.

#279 is a hotfix for this issue such that the value which corresponds to the side lying in the semiconductor is assigned to the grid point. Which is okay for calculating the drift of charge carriers inside the semiconductor.

However, in general this is not correct.

I think instead of assigning electric field vectors to all grid points of the electric potential,
we should assign electric field vectors to each volume between the grid points of the electric potential.
Thus, the grid of the electric field would be smaller by one in each dimension
as its ticks would be the midpoints of the ticks of the grid of the electric potential.

This would also require changes in the interpolation of the electric field vector during the drift.