Change GaudiConfig2 default semantics to use deepcopy of the default

This merge request changes the GaudiConfig2 DefaultSemantics so that it makes a deep copy of the default value.

This fixes the problem described in #114 (closed), but if a property with a non trivial type is only modified via its methods, we can only detect changes by comparing the default value with the stored value. Note that we still detect if the property is set to a new value equivalent to the default and handle the case correctly.

Closes #114 (closed)