Make RelationRange a proper view

[m-fila]

BEGINRELEASENOTES

• Make RelationRange fulfill the std::ranges::view and std::ranges::borrowed_range concepts

ENDRELEASENOTES

As suggested in #626 (comment) adding tests for checking whether RelationRange is a std range - it is.

Since the RelationRange is a very lightweight, non-owning view it can fulfill two more concepts that can be used by the std range algorithms and adapters to avoid unneeded operations:

• view - it's cheap to move/copy as it stores only iterators to the data living elsewhere
• borrowed_range - the iterators taken from range remain valid even when the range is destroyed since they are not local to the range

The RelationRange is very similar to std::span and std::ranges::subrange and could potential replaced by them except for two differences:

• RealationRange has at for bound-check element access whereas the alternatives don't (until C++26)
• RelationRange element access returns copy whereas the alternatives would return const reference. Since the elements are podio objects then copy is preferred.

The RelationRange could be also re-implemented with std::ranges::view_interface but it comes with issues like above so I think isn't really worth it

[tmadlener]

Nice. Thank you.