Document custom impact ionization

This model can be selected in the configuration file via the parameter `multiplication_model = "bologna"`.

## Custom Impact Ionization Models

Allpix Squared provides the possibility to use fully custom impact ionization models. In order to use a custom model, the

parameter `multiplication_model = "custom"` needs to be set in the configuration file. Additionally, the following

configuration keys have to be provided:

- `multiplication_function_electrons`:

  The formula describing the electron impact ionization gain.

- `multiplication_function_holes`:

  The formula describing the hole impact ionization gain.

The functions defined via these parameters can depend on the local electric field. In order to use the electric field

magnitude in the formula, an `x` has to be placed at the respective position.

Parameters of the functions can either be placed directly in the formulas in framework-internal units, or provided separately

as arrays via the `multiplication_parameters_electrons` and `multiplication_parameters_electrons`. Placeholders for

parameters in the formula are denoted with squared brackets and a parameter number, for example `[0]` for the first

parameter provided. Parameters specified separately from the formula can contain units which will be interpreted

automatically.

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Parameters directly placed in the impact ionization formula have to be supplied in framework-internal units since the

function will be evaluated with both electric field strength and doping concentration in internal units. It is recommended

to use the possibility of separately configuring the parameters and to make use of units to avoid conversion mistakes.

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It should be noted that the temperature passed via the module configuration is not evaluated for the custom impact ionization

model, but the model parameters need to be manually adjusted to the required temperature.

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The interpretation of the custom impact ionization functions is based on the `ROOT::TFormula` class \[[@rootformula]\] and

supports all corresponding features, mathematical expressions and constants.

[@massey]: https://doi.org/10.1109/TED.2006.881010

[@overstraeten]: https://doi.org/10.1016/0038-1101(70)90139-5

[@okuto]: https://doi.org/10.1016/0038-1101(75)90099-4