[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
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;