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.

dune/common/simd.hh

@@ -23,7 +23,7 @@ namespace Dune
   }
 
   template<typename T, std::size_t N, typename V, std::size_t M>
-  Vc::SimdArray<T,N,V,M> cond(const Vc::SimdMaskArray<T,N,V,M> & b,
+  Vc::SimdArray<T,N,V,M> cond(const typename Vc::SimdArray<T,N,V,M>::mask_type & b,
     const Vc::SimdArray<T,N,V,M> & v1,
     const Vc::SimdArray<T,N,V,M> & v2)
   {