Enhance FaustCoreCpp initialization by avoiding to create an empty/useless...

Enhance FaustCoreCpp initialization by avoiding to create an empty/useless Faust::TransformHelper() into the default constructor.

- In that purpose, the transform attribute reference has been replaced by a pointer. Because a reference can't be let uninitialized at construct stage (compil. error otherwise) and we unfortunately need to create an empty FaustCoreCpp object at initialization (for Cython FaustCorePy.pyx). Besides, it avoids the initialization through a copy constructor (here just one pointer is copied), this way the source object is kept and not lost uselessly.
- Defining in consequence a destructor in FaustCoreCpp.