Draft: Refactory 2020 env2

https://gitlab.ikp.kit.edu/AirShowerPhysics/corsika/-/wikis/Refactory2020

• ./Environment/UniformMagneticField.h
• ./Environment/Environment.h
• ./Environment/NuclearComposition.h
• ./Environment/DensityFunction.h RP
• ./Environment/NoMagneticField.h (Does not exist)
• ./Environment/IMediumModel.h RP
• ./Environment/HomogeneousMedium.h RP
• ./Environment/IMagneticFieldModel.h
• ./Environment/ShowerAxis.h
• ./Environment/IMediumPropertyModel.h RP
• ./Environment/IRefractiveIndexModel.h RP
• ./Environment/MediumProperties.h
• ./Environment/BaseExponential.h RP