Add sentinel support to IteratorRange

As of C++20, range based for supports that the end iterator has a different type. This allows to implement so called sentinels tagging the end. This can make it simpler and more efficient to implement ranges where it's not straight forward to get the end in O(1) or where comparing real iterators is expensive. To support this approach, this adds support for using a different type for the end iterator to Dune::IteratorRange.