[IteratorRange][Release] Make IteratorRange default-constructible and copyable

IteratorRanges are convenient for storing ranges of iterators into a
larger container, e.g. for the per-codim GeometryTypes in an
IndexSet. Unfortunately, you can't just create an array of
IteratorRanges right now because they can neither be copied, nor are
they default-constructible.

This patch fixes that problem and also adds a second typedef
const_iterator to make IteratorRange work in contexts where the
iterating code isn't using range-based for and assumes a const object.

dune/common/iteratorrange.hh

@@ -23,12 +23,22 @@ namespace Dune {
     //! The iterator belonging to this range.
     typedef Iterator iterator;
 
+    //! The iterator belonging to this range.
+    /**
+     * This typedef is here mainly for compatibility reasons.
+     */
+    typedef Iterator const_iterator;
+
     //! Constructs an iterator range on [begin,end).
     IteratorRange(const Iterator& begin, const Iterator& end)
       : _begin(begin)
       , _end(end)
     {}
 
+    //! Default constructor, relies on iterators being default-constructible.
+    IteratorRange()
+    {}
+
     //! Returns an iterator pointing to the begin of the range.
     iterator begin() const
     {
@@ -43,8 +53,8 @@ namespace Dune {
 
   private:
 
-    const Iterator _begin;
-    const Iterator _end;
+    Iterator _begin;
+    Iterator _end;
 
   };