[WIP] Add applyEntrywise method

With this tool, one can write quite general code to apply x = op(y) entrywise for vectors x, y (where y can indeed be a pack of vectors).

dune/matrix-vector/test/CMakeLists.txt

 dune_add_test(SOURCES arithmetictest.cc)
+dune_add_test(SOURCES genericvectortoolstest.cc)
 dune_add_test(SOURCES resizetest.cc)
 dune_add_test(SOURCES staticmatrixtoolstest.cc)
 dune_add_test(SOURCES triangularsolvetest.cc)