[referenceelements] Make reference element comparable and hashable

The identity of a reference element is for now based around the notion
that the implementation is a singleton, so comparisons are based on
the stored pointer value.

dune/geometry/referenceelement.hh

@@ -236,6 +236,25 @@ namespace Dune {
 
 #endif // DOXYGEN
 
+
+      //! Compares for equality with another reference element.
+      bool operator==(const ReferenceElement& r) const
+      {
+        return _impl == r._impl;
+      }
+
+      //! Compares for inequality with another reference element.
+      bool operator!=(const ReferenceElement& r) const
+      {
+        return not (*this == r);
+      }
+
+      //! Yields a hash value suitable for storing the reference element a in hash table
+      friend std::size_t hash_value(const ReferenceElement& r)
+      {
+        return reinterpret_cast<std::size_t>(r._impl);
+      }
+
     private:
 
       // The implementation must be a friend to construct a wrapper around itself.

dune/geometry/test/test-referenceelements.cc

@@ -34,6 +34,15 @@ int main () try
 
   type.makeLine();
 
+  {
+    // check default constructibility and comparison operators
+    ReferenceElements<double,1>::ReferenceElement r1, r2;
+    test(r1 == r2);
+    test(not (r1 != r2));
+    // check hash value
+    testcmp(hash_value(r1),0);
+  }
+
   const ReferenceElement<double,1>& referenceLine = ReferenceElements<double, 1>::general(type);
 
   // size(int c)