Change include guards to #pragma once.

COAST/COASTProcess.h

COAST/COASTStack.h

COAST/ParticleConversion.h

Environment/BaseExponential.h

Environment/DensityFunction.h

Environment/Environment.h

Environment/FlatExponential.h

Environment/HomogeneousMedium.h

Environment/IMediumModel.h

Environment/InhomogeneousMedium.h

Environment/LinearApproximationIntegrator.h

Environment/NameModel.h

Environment/NuclearComposition.h

Environment/SlidingPlanarExponential.h

Environment/VolumeTreeNode.h

Framework/Cascade/Cascade.h

Framework/Cascade/testCascade.h

Framework/Geometry/BaseTrajectory.h

Framework/Geometry/BaseVector.h

Framework/Geometry/CoordinateSystem.h