Allow Different Sizes for the Electric Field

This MR fixes issue #96 (closed) and allows the usage of different field sizes. Up till now, only electric fields of a single unit cell could be used. This MR adds a new parameter, called field_scale to the ElectricFieldReader module which allows to define the fraction of a pixel which the given field occupies.

As an example, it is very common to only simulate a quarter of a pixel unit cell in TCAD (half-pitch in both directions). These fields can now be used by setting

[ElectricFieldReader]
model = "init"
field_scale = 0.5 0.5

The field is scaled into 0.5 of the pixel cell and mirrored at the border to satisfy periodic boundary conditions. Another possibility is that the pixel implants and thus the electric field are different e.g. for even and odd columns of the chip. In this case, the full TCAD simulation would span multiple pixels, e.g. a 2x2 pixel grid. This field can now be used by setting:

[ElectricFieldReader]
model = "init"
field_scale = 2.0 2.0

in the configuration of the simulation.

The implementation of this feature is transparent to the current approach where the field is always scaled to one pixel, i.e. field_scale defaults to 1.0 1.0. In case of a mismatch between the size of the electric field read from the INIT file and the expected field are (calculated as pitch times the scale factors), a warning is printed but the simulation continues with a field matched to the configured size, ignoring the size of the original field indicated in the file.

This fixes #96 (closed)