Destructors for generated Obj classes should be defaulted if there are no relations

[tmadlener]

Currently the ~Obj destructor is generated unconditionally and is defined as (sketched)

~Obj() {
  // deal with OneToManyRelations and VectorMembers
  // deal with OneToManyRelations
}

Hence, if there are no relations defined for a datatype it could simply be declared as

virtual ~Obj() = default;

instead of implementing an empty destructor.

[tmadlener]

The assignment operator for the user facing classes could make use of the "copy-and-swap" idiom.