fixed multiprocessing issue for now -> newest running code in Population_evolution_OwWu17__04_06-2020_FAST.ipynb

updated Beta_K_functions, Planet_models_LoFo14, Planet_models_Ot20 and Mass_evolution_function to be compliant with PEP8 formatting standards.

I made changes to Planet_class_LoFo14, in particular the output files, then I had to fix some issues with time steps in Lx_evo for the OwWu17 Lx track with const. slope, and I was in mass_evolution_function. These were some important fixes, now I can run the OwWu17 sample on merlot. YAY!