Support of GaAs:Cr material

An experimental version of GaAs:Cr sensor material support [article about experimental verification is accepted].

Ruch-Kino model for electron mobility was added [B. Bergmann et al. 2020 JINST 15 C03013]. Linear dependence of hole mobility on the electric field was implemented. Recombination of charge carriers is very simple -- just through corresponding lifetimes since we do not know properly the concentration of defects in the material.

For Transient and Generic propagation modules the following lines should be added to config:

mobility_model = "ruch_kino_gaas"
recombination_model = "lifetimes_gaas"
electron_lifetime = 20ns
hole_lifetime = 4.5ns

In DepositionGeant4 module one should add Fano factor and e-h pair creation energy for GaAs:

fano_factor = 0.14
charge_creation_energy = 4.2eV

Added "timepix3_GaAs" detector model, where the sensor material is specified as well as the material and thickness of the common electrode. Actually, it's a copy of "timepix" model, but chip thickness was decreased down to 300um (not always it is true).

In ElectricField module the direction in Z for constant electric field was changed.

In PulseTransfer module the pseudo pulse generation was changed (only for Constant field): at negative bias voltages pulse should have negative polarity.