add using std::sqrt

In the last time I stumbled over error messages like

error: no matching function for call to ‘sqrt(double)‘

a lot. The problem is that loop.hh in dune-common, defines the cmath functions in the Dune namespace, which are the found by the unqualified name lookup before the declarations in the namespace std. This merge request adds using directives everywhere sqrt is used to avoid this problem. Additionally it probably removes the name qualifier std:: before sqrt.