[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/genericvectortoolstest.cc

@@ -16,11 +16,6 @@
 
 using namespace Dune;
 
-//template<typename X, typename Y>
-//bool my_or(const X& x, const Y& y) {
-  //return x || y;
-//}
-
 TestSuite test_inplace_or() {
   TestSuite suite;