hp::Collection: allow range-based loop

[peterrum]

... just like in the case of VectorizedArray.

[marcfehling]

I like this!

Your test is about FECollection objects. For a moment I thought that a test for MappingCollection objects would be nice, but it's really not necessary as the both inherit from hp::Collection.

[marcfehling]

doxygen complains:

/home/runner/work/dealii/dealii/include/deal.II/hp/collection.h:39: warning: argument 'lane' of command @param is not found in the argument list of hp::CollectionIterator< T >::CollectionIterator(const std::vector< std::shared_ptr< const T >> &data, const std::size_t index)
/home/runner/work/dealii/dealii/include/deal.II/hp/collection.h:44: warning: The following parameters of hp::CollectionIterator::CollectionIterator(const std::vector< std::shared_ptr< const T >> &data, const std::size_t index) are not documented:
  parameter 'index'

[bangerth]

Yes, very nice! I had briefly looked at this after you had mentioned the issue in a comment, but found that we store a collection of pointers and that iterating over them wasn't going to be as easy as iterating over the pointers because we want to get back the objects. Nice work encoding this in your CollectionIterator class :-)

/rebuild

[peterrum]

/rebuild