Fix compile warning by adding copy constructor to hp iterator

[kronbichler]

My compiler (clang-13) complained about

/home/kronbichler/deal/deal.II/include/deal.II/hp/collection.h:80:5: warning: definition of implicit 
copy constructor for 'CollectionIterator<dealii::FiniteElement<1>>' is deprecated because it has a 
user-declared copy assignment operator [-Wdeprecated-copy]
    operator=(const CollectionIterator<T> &other) = default;

So we better add a copy constructor as well. Alternatively, we could have relied on the implicitly generated copy assignment operator.