made leap-frog more stabel and robust, also in extrem magnetic field cases

Documentation/Examples/boundary_example.cc

Documentation/Examples/cascade_example.cc

Documentation/Examples/cascade_proton_example.cc

Documentation/Examples/em_shower.cc

Documentation/Examples/hybrid_MC.cc

Documentation/Examples/vertical_EAS.cc

Environment/BaseExponential.h

Environment/DensityFunction.h

Environment/FlatExponential.h

Environment/HomogeneousMedium.h

Environment/IMediumModel.h

Environment/InhomogeneousMedium.h

Environment/LinearApproximationIntegrator.h

Environment/ShowerAxis.cc

Environment/ShowerAxis.h

Environment/SlidingPlanarExponential.h

Environment/testEnvironment.cc

Environment/testShowerAxis.cc

Framework/Cascade/Cascade.h

Framework/Geometry/Trajectory.h