[!551] [SIMD] Provide ostream inserters.

Merge branch 'simd-io' into 'master'

Apparently, output of Vc vectors does not work with libc++. I suspect it has
to do with some hackery Vc does in relation to the standard output streams.

This introduces syntax like

      std::cout << Dune::Simd::io(simd_vec) << std::endl;
      std::cout << Dune::Simd::vio(simd_vec) << std::endl;

to produce output like

    <1, 2, 3, 4>

The difference between the two versions is that io() will skip the angle
bracket for one-component vectors, while vio() will always print them.

See merge request [!551]

  [!551]: gitlab.dune-project.org/core/dune-common/merge_requests/551