Add initial versions of GUrl and GUrlFile classes.

The GUrl class is an abstract base class that defines the interface for URL access classes.

The GUrlFile class implements handling of file URLs.
Add GUrl and GUrlFile classes.
Add documentation.
Ignore test/test_url.dat
Add GUrl and GUrlFile classes.
Add GUrl and GUrlFile classes.

Due to interface problems, the printf() and scanf() methods have not been implemented. The read() and write() methods have been implemented differently.
Add GUrlFile test.
Add information about GUrl and GUrlFile classes.