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/conditional.hh

+#ifndef DUNE_COMMON_CONDITIONAL_HH
+#define DUNE_COMMON_CONDITIONAL_HH
+
+namespace Dune
+{
+
+    /** \brief conditional evaluate
+
+        sometimes call immediate if, evaluates to
+
+        \code
+        if (b)
+           return v1;
+        else
+           return v2;
+        \endcode
+
+        In contrast to if-then-else the cond function can also be
+        evaluated for vector valued SIMD data types, see simd.hh.
+
+        \param b boolean value
+        \param v1 value of b==true
+        \param v2 value of b==false
+    */
+    template<typename T1, typename T2>
+    const T1 cond(bool b, const T1 & v1, const T2 & v2)
+    {
+        return (b ? v1 : v2);
+    }
+
+} // end namespace Dune
+
+#endif // DUNE_COMMON_CONDITIONAL_HH