Add constructor from std::initializer_list

With this, you can now write

Dune::Solvers::Interval<double> foo = {-1,1};

The patch also adds the default constructor, which is not automatically
created anymore.

dune/solvers/common/interval.hh

@@ -18,6 +18,15 @@ template <class field_type>
 class Interval
 {
 public:
+    /** \brief Default constructor */
+    Interval()
+    {}
+
+    /** \brief Construct from an initializer list */
+    Interval(std::initializer_list<field_type> const &input)
+    : data_(input)
+    {}
+
     /** \brief Array-like access
      */
     field_type& operator[](int i)