arviz layer client: adding size() method

source/RobotAPI/components/ArViz/Client/Layer.h

@@ -46,6 +46,11 @@ namespace armarx::viz
             (add(std::forward<Ts>(elems)), ...);
         }
 
+        std::size_t size() const noexcept
+        {
+            return data_.elements.size();
+        }
+
         data::LayerUpdate data_;
     };