Add copy assignment to ecs::accessor

libs/ecs/include/psemek/ecs/accessor.hpp

@@ -14,10 +14,14 @@ namespace psemek::ecs
 		 */
 		accessor() = default;
 
-		/** Copy the accessor.
+		/** Copy-construct the accessor.
 		 */
 		accessor(accessor const &) = default;
 
+		/** Copy-assign the accessor.
+		 */
+		accessor & operator = (accessor const & other) = default;
+
 		/** Check if this accessor is valid, i.e. refers to an entity.
 		 *
 		 *  @return True if the accessor is valid, false otherwise