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.

cmake/modules/AddVcFlags.cmake

+# Defines the functions to use VC
+#
+# .. cmake_function:: add_dune_vc_flags
+#
+#    .. cmake_param:: targets
+#       :positional:
+#       :single:
+#       :required:
+#
+#       A list of targets to use VC with.
+#
+
+function(add_dune_vc_flags _targets)
+  if(Vc_FOUND)
+    foreach(_target ${_targets})
+      target_link_libraries(${_target} ${Vc_LIBRARIES})
+      target_compile_options(${_target} PUBLIC ${Vc_ALL_FLAGS})
+      target_include_directories(${_target} SYSTEM PUBLIC ${Vc_INCLUDE_DIR})
+    endforeach(_target ${_targets})
+  endif(Vc_FOUND)
+endfunction(add_dune_vc_flags)
+
+if(Vc_FOUND)
+  dune_register_package_flags(COMPILE_OPTIONS "${Vc_ALL_FLAGS}"
+                              LIBRARIES "${Vc_LIBRARIES}"
+                              INCLUDE_DIRS "${Vc_INCLUDE_DIR}")
+endif(Vc_FOUND)
+set(HAVE_VC ${Vc_FOUND})