fix: Fit iterator traits for ContainerIndexIterator (#3689)

It looks like that `std:: iterator_traits< Acts::ContainerIndexIterator< whatever ...> >::iterator_category ` was evaluated to `std::output_iterator_tag` instead of `Iter::iterator_category` (i.e. `std::random_access_iterator_tag`). This made it impossible to use `std::distance` with this iterator class.

See https://en.cppreference.com/w/cpp/iterator/iterator_traits  to see why.

This PR fixes it.

Core/include/Acts/Utilities/Iterator.hpp

@@ -22,6 +22,8 @@ class ContainerIndexIterator {
   using container_type =
       std::conditional_t<Const, const _Container, _Container>;
   using difference_type = std::ptrdiff_t;
+  using pointer = void;
+  using reference = void;
 
   ContainerIndexIterator() : m_container(nullptr), m_index(0) {}