Add alternative growth rate formulae

[olaszsoma]

The growth rate calculations should be updated with an option to use the most recent formulas for Dreicer and avalanche generations. These would include effects from impurities and radiations and were developed by the runaway team at Chalmers. The formula for the new avalanche growth rate should be found and linked here as well.

[olaszsoma]

The generalization of the avalanche growth rate was published by Hesslow, et. al. (Hesslow, L., Embréus, O., Vallhagen, O. & Fülöp, T. 2019 Influence of massive material injection on avalanche runaway generation during tokamak disruptions. Nuclear Fusion 59 (8), 084004.).

[olaszsoma]

Have to check how to call Matlab from C++ codes.

[olaszsoma]

The neural network for the updated Dreicer generation rate was implemented in c++ for the DREAM code. This could be used as a base for the implementation in Runaphys.