Merge branch 'add_using_directives' into 'master'

ref:core/dune-geometry\> This MR adds using directives for the use of
std::sqrt or std::abs. This is needed because other names in the namespace
Dune are probably found before the names in the namespace std.

See merge request [!139]

  [!139]: gitlab.dune-project.org/core/dune-geometry/merge_requests/139