Added example for constant and regions models. Corrected units in tests.

* `output_plots_single_pixel`: Determines if the whole sensor has to be plotted or only a single pixel. Defaults to true (plotting a single pixel).

## Usage

In the following example a doping profile map in APF format is loaded:

```ini

[DopingProfileReader]

model = "mesh"

file_name = "example_doping_profile.apf"

field_mapping = PIXEL_FULL

```

In the next example two regions with constant doping are used. Down to a depth of `25um` the doping concentration is $`6\times 10^{11} \,\text{cm}^{-3}`$, while below it the doping is set to $`5\times 10^{18} \,\text{cm}^{-3}`$:

```ini

[DopingProfileReader]

model = "regions"

doping_concentration = [[25um,6e11/cm/cm/cm],[50um,5e18/cm/cm/cm]]

```

Finally, a constant doping of $`-2.13\times 10^{15} \,\text{cm}^{-3}`$ is set for the entire sensor:

```ini

[DopingProfileReader]

model = "constant"

doping_concentration = -2.13e+15/cm/cm/cm

```

[DopingProfileReader]

log_level = DEBUG

model = "regions"

doping_concentration = [[20um, 100000000000], [100um, -3000000000000]]

doping_concentration = [[20um, 1e14/cm/cm/cm], [100um, -3e15/cm/cm/cm]]

#PASS Set constant doping concentration of -3e+15/cm/cm/cm at sensor depth 100um

[DopingProfileReader]

log_level = DEBUG

model = "constant"

doping_concentration = -2134560000000

doping_concentration = -2.13456e+15/cm/cm/cm

#PASS Set constant doping concentration of -2.13456e+15/cm/cm/cm