Define ClientData::Site::size

src/ClientData.h

@@ -217,6 +217,8 @@ public:
    Site& operator =( Site && other )
       { mData = std::move(other.mData); return *this; }
 
+   size_t size() const { return mData.size(); }
+
    /// \brief a type meant to be stored by client code in a static variable,
    /// and used as a retrieval key to get the manufactured client object back
    /// from the host object.