Added license text to all old header files where it was missing.

FileIO/Gmsh2GeoIO.cpp

FileIO/Gmsh2GeoIO.h

FileIO/Legacy/OGSIOVer4.cpp

FileIO/Legacy/OGSIOVer4.h

FileIO/MeshIO/GMSHAdaptiveMeshDensity.cpp

FileIO/MeshIO/GMSHAdaptiveMeshDensity.h

FileIO/MeshIO/GMSHFixedMeshDensity.cpp

FileIO/MeshIO/GMSHFixedMeshDensity.h

FileIO/MeshIO/GMSHInterface.cpp

FileIO/MeshIO/GMSHInterface.h

FileIO/MeshIO/GMSHMeshDensityStrategy.h

FileIO/MeshIO/GMSHPoint.cpp

FileIO/MeshIO/GMSHPoint.h

FileIO/MeshIO/TetGenInterface.cpp

FileIO/MeshIO/TetGenInterface.h

FileIO/readNonBlankLineFromInputStream.cpp

FileIO/readNonBlankLineFromInputStream.h

GeoLib/PointWithID.h

GeoLib/Polygon.cpp

GeoLib/Polygon.h