Add an ordering to hybrid tree paths even for multiple types using the three way comparison operator from C++20.
Note that while the result is constexpr, the three way operator does not generate hybrid friendly comparison operators. I think this is more a question for dune-common: How to treat the ordering from the three way comparison operator for Dune::Hybrid algorithms?
TODO:
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Document the new functionality (important: explain when it is guaranteed to be totally ordered wrt to other types)