Replace TreePath by HybridTreePath

Carsten Gräserrequested to merge
carsten.graeser/dune-pdelab:feature/compatible-hybridtreepath into master

This replaces all uses of TreePath<i...> by HybridTreePath<Dune::index_constant<i>...>. Notice that this will only work with staging/dune-typetree!69 (merged).

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