Merge branch 'p-transient-readme' into 'master'

* `max_charge_groups`: Maximum number of charge groups to propagate from a single deposit point. Temporarily increases the value of `charge_per_step` to reduce the number of propagated groups if the deposit is larger than the value `max_charge_groups`*`charge_per_step`, thus reducing the negative performance impact of unexpectedly large deposits. The default value is 1000 charge groups. If it is set to 0, there is no upper limit on the number of charge groups propagated.

* `timestep`: Time step for the Runge-Kutta integration, representing the granularity with which the induced charge is calculated. Default value is 0.01ns.

* `integration_time`: Time within which charge carriers are propagated. After exceeding this time, no further propagation is performed for the respective carriers. Defaults to the LHC bunch crossing time of 25ns.

* `distance`: Maximum distance of pixels to be considered for current induction, calculated from the pixel the charge carrier under investigation is below. A distance of `1` for example means that the induced current for the closest pixel plus all neighbors is calculated. It should be noted that the time required for simulating a single event depends almost linearly on the number of pixels the induced charge is calculated for. Usually, a 3x3 grid (9 pixels, distance 1) should suffice since the weighting potential at a distance of more than one pixel pitch often is small enough to be neglected while the simulation time is almost tripled for `distance = 2` (5x5 grid, 25 pixels).

* `distance`: Maximum distance of pixels to be considered for current induction, calculated from the pixel the charge carrier under investigation is below. A distance of `1` for example means that the induced current for the closest pixel plus all neighbors is calculated. It should be noted that the time required for simulating a single event depends almost linearly on the number of pixels the induced charge is calculated for. Usually, for Cartesian sensors a 3x3 grid (9 pixels, distance 1) should suffice since the weighting potential at a distance of more than one pixel pitch often is small enough to be neglected while the simulation time is almost tripled for `distance = 2` (5x5 grid, 25 pixels). To just calculate the induced current in the one pixel the charge carrier is below, `distance = 0` can be used. Defaults to `1`.

* `ignore_magnetic_field`: The magnetic field, if present, is ignored for this module. Defaults to false.

* `multiplication_model`: Model used to calculate impact ionization parameters and charge multiplication. Defaults to `none` which corresponds to unity gain, a list of available models can be found in the documentation.

* `multiplication_threshold`: Threshold field above which charge multiplication is calculated. Defaults to `100kV/cm`.