some density modelling refactoring

corsika/detail/media/BaseExponential.inl

corsika/detail/media/FlatExponential.inl

corsika/detail/media/HomogeneousMedium.inl

corsika/detail/media/InhomogeneousMedium.inl

corsika/detail/media/LayeredSphericalAtmosphereBuilder.inl

corsika/detail/media/LinearApproximationIntegrator.inl

corsika/detail/media/SlidingPlanarExponential.inl

corsika/detail/modules/energy_loss/BetheBlochPDG.inl

corsika/detail/modules/proposal/ContinuousProcess.inl

corsika/media/BaseExponential.hpp

corsika/media/FlatExponential.hpp

corsika/media/HomogeneousMedium.hpp

corsika/media/IMediumModel.hpp

corsika/media/InhomogeneousMedium.hpp

corsika/media/LayeredSphericalAtmosphereBuilder.hpp

corsika/media/LinearApproximationIntegrator.hpp

corsika/media/SlidingPlanarExponential.hpp

tests/media/testEnvironment.cpp

tests/media/testMagneticField.cpp

tests/media/testMedium.cpp