Correct documentation error.
Update documentation.
Rename add() method to merge() and improve documentation.

The add() method has been renamed to merge() to make clear that this method performs a merging of intervals.

The Doxygen documentation has been improved, syntax errors have been corrected, and the language usage is more concise.
Make GOptimizerPars class abstract.

For some reason, the GOptimizerPars class was not abstract although it should be an abstract class. The code has been modified to make the class now abstract.
Update documentation.
Use GContainer as base class.

GOptimizerPars is a container class, hence we derive it now from GContainer.
Make class compliant to container class interface.

GModels derives from GOptimizerPars, which is now a container class. GModels needs therefore to implement all container class methods. Methods have been added to insert and remove models, and to extend the model container by another model container.
Derive GOptimizerPars and GModels from GContainer.