[smart pointers] private class auto delete management

[mathildemerle]

After discussion with @Florent2305 it could be interesting to switch the creation of private class to QScopedPointer or something else, avoiding the need to do delete d; as done in some process in medInria.

It still needs deletion of variables created by new in these private classes.

Cf medAbstractParameter::~medAbstractParameter() comment, empty destructor of the class (or default) is needed.