add support for vectorization library Vc

This branch adds support for the Vc library, which add support for vectorization, including an abstraction for intrisics.

We have been using in EXA-Dune and I belive that the integration of vectorization support into the core-Dune is something Dune in general can benefit from. A separate branch I'm preparing for ISTL will add support for multiple RHS vectors using vectorization.

The branch collects a set of helper classes which allow to add vectorization support to your code with moderate effort. Mainly this means that we have a couple of utility functions, which work for scalar data types, ranges and SIMD data types alike.

See also: !16 (merged), dune-istl!17 (merged), dune-geometry!13 (closed).

Note: This is the followup merge request to !16 (merged). The original was merged accidentially and had to be reverted, see #26 (closed). The branch had to be rebased; if you had checked out an earlier version of feature/vc you may need to take special action to rebase onto the new branch.#