[performance][math] Replace std::pow(Scalar, int) with Dune::power(Scalar, int) or write product explicitly

A quick grep showed that we use std::pow(Scalar, int) at quite a lot of places. Performance could be improved by using Dune::power(Scalar, int) (which is even constexpr) for exponents > 2 or just by explicitly writing down the product.

Scalar result0 = std::pow(a, 2);

Scalar result1 = Dune::power(a, 2);

Scalar result2 = a*a;