Refactory 2020 env1

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

• ./Environment/SlidingPlanarExponential.h
• ./Environment/LinearApproximationIntegrator.h
• ./Environment/MediumPropertyModel.h
• ./Environment/UniformRefractiveIndex.h
• ./Environment/LayeredSphericalAtmosphereBuilder.h
• ./Environment/FlatExponential.h
• ./Environment/VolumeTreeNode.h
• ./Environment/MediumTypes.h
• ./Environment/InhomogeneousMedium.h
• ./Environment/NameModel.h