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

@@ -8,8 +8,8 @@
 namespace Dune
 {
 
-  template<typename T>
-  const T & cond(bool b, const T & v1, const T & v2)
+  template<typename T1, typename T2>
+  const T1 & cond(bool b, const T1 & v1, const T2 & v2)
   {
     return (b ? v1 : v2);
   }