Simplify implementation for vectors with scalar entries

Using the new Impl::asVector() from dune-common we can get rid of

• several Hybrid::isElse() constructions used to implement special cases for isNumber,
• undefined behaviour invoked by toVector() due to the latter violating strict aliasing rules.

This also removed the dangerous toVector() methods. This is only a partial solution to #63 (closed). We also need to do the same for toMatrix() which requires an analogue Impl::asMatrix() function and a ScalarMatrixView wrapper class.

dune/istl/bcrsmatrix.hh

@@ -20,6 +20,7 @@
 #include <dune/common/iteratorfacades.hh>
 #include <dune/common/typetraits.hh>
 #include <dune/common/ftraits.hh>
+#include <dune/common/hybridutilities.hh>
 
 /*! \file
  * \brief Implementation of the BCRSMatrix class