EFieldReader: parameter explanations in readme

- For **linear** electric fields, the field has a constant slope determined by the bias voltage and the depletion voltage.

  The sensor is depleted either from the implant or the back side, the direction of the electric field depends on the sign of

  the bias voltage (with negative bias voltage the electric field vector points towards the backplane and vice versa). If the

  sensor is depleted from the implant side, the electric field is calculated using the formula

  the bias voltage (with negative bias voltage the electric field vector points towards the backplane and vice versa).

  The sign of depletion_voltage is always ignored. 

  If the sensor is depleted from the implant side, the absolute value of the electric field is calculated using the formula

  ```math

  E(z) = \frac{U_{bias} - U_{depl}}{d} + 2 \frac{U_{depl}}{d}\left( 1- \frac{z}{d} \right),

  E(z) = \|frac{U_{bias}| - |U_{depl}|}{d} + 2 \frac{|U_{depl}|}{d}\left( 1- \frac{z}{d} \right),

  ```

  where d is the thickness of the sensor, and $`U_{depl}`$, $`U_{bias}`$ are the depletion and bias voltages, respectively.

  In case of a depletion from the back side, the electric field is calculated as

  In case of a depletion from the back side, the absolute value of the electric field is calculated as

  ```math

  E(z) = \frac{U_{bias} - U_{depl}}{d} + 2 \frac{U_{depl}}{d}\left( \frac{z}{d} \right).

  E(z) = \frac{|U_{bias}| - |U_{depl}|}{d} + 2 \frac{|U_{depl}|}{d}\left( \frac{z}{d} \right).

  ```

- For **parabolic** electric fields, a parabola is defined in order to emulate a double-peaked field such as the electric

  fields observed in sensors after irradiation. The parabola is calculated from the position $`z_{min}`$ and value