[cleanup] replace std::pow(Scalar, int) with Dune::power(Scalar,int)

Timo Kochrequested to merge
feature/dune-power into master

Remake of reverted merge request !2263 (merged)

  • Include the Dune header for power @timjupe

Closes #880 (closed)

Edited by Kilian Weishaupt

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