Move readmodel into cpp folder.
Properly format XML document for an element containing a single text leaf.

A element containing a single text leaf should be written in a single line without indentation. This special case is now detected and the proper formatting is performed.
Remove indentation from text formatting.

Text should never be indented in an XML file as text follows directly the tag end.
Add automatic GXmlNode output pointer typecasting.
Add XML example script.
Rename xml_create.py to xml_howto.py
Correctly ignore stuff.
Add carriage returns after text elements.
Added remove_attribute() method.

The remove_attribute() method allows removing of attributes from an XML element. Now the user has full control about adding, changing and removing attributes.
Review documentation.
Add initial example script.

This script creates a simple HTML document using GammaLib's xml module.
Remove unnecessary import.
Update ignore file after adding of examples.
Add autoscale() method.

The autoscale() method sets the scale factor so that the value becomes unity. This is only possible for parameters with non-zero values.
Add autoscale() method.

The autoscale() method performs auto-scaling for all parameters in the spectral model. Auto-scaling means that the scale factor is that so that the value is unity.
Remove autoscale() and perform auto-scaling in parameter constructor.

The autoscale() method has been removed as it exists now in the GModelSpectral() base class.

The parameter constructor now calls autoscale() to perform an autoscaling of all model parameters.
Add autoscale() method.
Initial simple example ... to be finished.
Preliminary implementation of new model parameter interface.

A new model parameter interface has been implemented that puts the internal workings of the class more in the background. For example, the real_xxx() methods have been replaced by Xxx() methods, and the xxx() methods became factor_xxx(). For example, real_value() becomes Value() and value() becomes factor_value(). For the moment, upper case letters are used so that no mix with the former xxx() methods occur. This is for validating the change, and will be modified in a future commit.
Adapt to new GModelPar interface.