support Directory Reference

Files
	GBilinear.cpp
	Bilinear interpolator class interface implementation.
	GCsv.cpp
	Comma-separated values table class implementation.
	GDaemon.cpp
	Daemon class implementation.
	GException.cpp
	Exception handler interface implementation.
	GFilename.cpp
	Filename class interface implementation.
	GNodeArray.cpp
	Node array class implementation.
	GRan.cpp
	Random number generator class implementation.
	GTools.cpp
	Gammalib tools implementation.
	GUrl.cpp
	GUrlFile.cpp
	File URL class interface implementation.
	GUrlString.cpp
	String URL class interface implementation.