TreeExecutionServer plugin loading order mismatch docstring vs implementation

[thettasch-samxl]

In the TreeExecutionServer, plugins are loaded in the following order: user plugins, then parameter based plugins. See the snippet below:

BehaviorTree.ROS2/behaviortree_ros2/src/tree_execution_server.cpp

Lines 116 to 119 in 6c6aa07

	// user defined method
	registerNodesIntoFactory(p_->factory);
	// load plugins from multiple directories
	RegisterPlugins(p_->params, p_->factory, node_);

The docstring for registerNodesIntoFactory, on the other hand, claims that it is called only after the other plugins were registered:

BehaviorTree.ROS2/behaviortree_ros2/include/behaviortree_ros2/tree_execution_server.hpp

Lines 104 to 112 in 6c6aa07

	/**
	* @brief registerNodesIntoFactory is a callback invoked after the
	* plugins were registered into the BT::BehaviorTreeFactory.
	* It can be used to register additional custom nodes manually.
	*
	* @param factory The factory to use to register nodes
	*/
	virtual void registerNodesIntoFactory(BT::BehaviorTreeFactory& factory)
	{}

So either the docstring or the implementation is wrong.

Personally, I would prefer if the order was switched in the implementation, so that registerNodesIntoFactory runs last. Then I could use it as a hook to do other things (e.g. export an XSD schema) right after all the plugins are loaded.

I don't mind contributing to fix the issue, I'd just need an indication as to which should be fixed, the implementation or the docstring,