fix clang support

see https://gitlab.dune-project.org/pdelab/dune-pdelab/-/jobs/142529/raw

particular issues:

/builds/pdelab/dune-pdelab/dune/pdelab/constraints/hangingnode.hh:79:30: error: no matching constructor for initialization of 'const Dune::GeometryType'
          const GeometryType vertex_gt(0);

I guess it should be sufficient to replace vertex_gt(0) by Dune::GeometryTypes::vertex

/builds/pdelab/dune-pdelab/doc/Recipes/recipe-linear-system-assembly.cc:166:27: error: constexpr variable 'nonzeros' must be initialized by a constant expression
/builds/pdelab/dune-pdelab/doc/Recipes/recipe-linear-system-solution-istl.cc:166:27: error: constexpr variable 'nonzeros' must be initialized by a constant expression
/builds/pdelab/dune-pdelab/doc/Recipes/recipe-geometry-grid.cc:175:27: error: constexpr variable 'nonzeros' must be initialized by a constant expression
/builds/pdelab/dune-pdelab/doc/Recipes/recipe-linear-system-solution-pdelab.cc:161:27: error: constexpr variable 'nonzeros' must be initialized by a constant expression

This is all the same error and I think this is really a bug. The current implementation uses std::pow to calculate the nonzeros, but how could std::pow be constexpr? We can instead use Dune::power.