[SIMD] Provide ostream inserters.

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.

enabled an automatic merge when the pipeline for b7598f60 succeeds

merged

mentioned in commit 0ff1384d

mentioned in commit dune-istl@935fd43f

mentioned in merge request dune-istl!223 (merged)

mentioned in commit dune-istl@c412d83a