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 For example, the real_xxx() methods have been replaced by Xxx() methods, and the xxx() methods became factor_xxx().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.
Adapt to new GModelPar interface.

The GModelPar interface has been revisited and the code has been adapted to satisfy the new interface.
Add changes in GModelPar and GXml and GXmlNode classes
Correct optimization bug.

The GObservations::copy_members() copied the m_fct which it should not have done, as the m_fct::m_this member shall always point to GObservations, hence it should never be copied! Copying led in fact to a segmentation fault. The copy instruction has been removed which corrected the bug.