Revise GFitsTable class interface.

A major revision of the GFitsTable class interface has been done, making it more similar to the interface of other container classes. In particular, the column access operator now returns a pointer instead of a reference, preventing code slicing. The set() method has been introduced to set columns in a table. The append_column() and insert_column() methods have been renamed to append() and insert() as they act on columns, and a table is a column container. Some arguments or return values are now also declared as const references. A size() method has been added that return the number of table columns.

As the GFitsTable class is widely used throughout GammaLib, a number of classes had to be adapted to this new interface.
Add XML documentation and example file.