Add the TupleVector class from dune-functions to dune-solvers

Because it is useful, e.g., to handle the bit vectors corresponding to MultiTypeBlockVector
vectors.  The same class already exists in dune-functions, which may lead to confusion.
In the long run, both implementations should be replaced by a single one in a module
further upstream.

dune/solvers/common/CMakeLists.txt

@@ -11,4 +11,5 @@ install(FILES
     preconditioner.hh
     resize.hh
     staticmatrixtools.hh
+    tuplevector.hh
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/solvers/common)

dune/solvers/common/tuplevector.hh

+// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+// vi: set et ts=4 sw=2 sts=2:
+#ifndef DUNE_SOLVERS_COMMON_TUPLEVECTOR_HH
+#define DUNE_SOLVERS_COMMON_TUPLEVECTOR_HH
+
+#include <tuple>
+
+#include <dune/typetree/utility.hh>
+
+namespace Dune
+{
+namespace Solvers
+{
+
+  /**
+   * \brief A std::tuple that allows access to its element via operator[]
+   */
+  template<class... T>
+  class TupleVector : public std::tuple<T...>
+  {
+    using Base = std::tuple<T...>;
+
+  public:
+
+    /** \brief Construct from a set of arguments */
+    template<class... TT>
+    constexpr TupleVector(TT&&... tt) :
+      Base(std::forward<TT>(tt)...)
+    {}
+
+    /** \brief Default constructor */
+    constexpr TupleVector()
+    {}
+
+    /** \brief Const access to the tuple elements */
+    template<std::size_t i>
+    auto operator[](const Dune::TypeTree::index_constant<i>&) const
+      ->decltype(std::get<i>(*this))
+    {
+      return std::get<i>(*this);
+    }
+
+    /** \brief Non-const access to the tuple elements */
+    template<std::size_t i>
+    auto operator[](const Dune::TypeTree::index_constant<i>&)
+      ->decltype(std::get<i>(*this))
+    {
+      return std::get<i>(*this);
+    }
+
+    /** \brief Number of elements of the tuple */
+    static constexpr std::size_t size()
+    {
+      return std::tuple_size<Base>::value;
+    }
+
+  };
+
+}  // namespace Functions
+
+}  // namespace Dune
+
+#endif
\ No newline at end of file