resource loaning

[Karsten1987]

Part four of #164

currently sits on top of #207

[Karsten1987]

I've added a set of function overloads in 7ac53d5 which lets the user add hardware components which are not declared in the URDF.
This is mainly with respect to the gazebo_ros2_control packages, which I believe create a hardware components dynamically based on the original URDF (not the ros2_control tags). There's no straightforward way for me to figure out how a gazebo plugin can be loaded twice, once as a hardware component as well as a gazebo plugin to use gazebo API.

[Karsten1987]

I hope to have addressed all review comments @bmagyar

[Karsten1987]

@destogl please go ahead and open follow-up tickets or continue to comment on this PR in case you have outstanding comments.

[destogl]

I don't have major comments, just need a few clarifications. For example, I am missing unclaiming of the command_interfaces.

[Karsten1987]

The "unclaiming" happens implicit in the destructor of the LoanedCommandInterface, here:

ros2_control/hardware_interface/src/resource_manager.cpp

Lines 212 to 216 in 925f5f3

	void ResourceManager::release_command_interface(const std::string & key)
	{
	std::lock_guard<decltype(resource_lock_)> lg(resource_lock_);
	claimed_command_interface_map_[key] = false;
	}

The destructor has a support for a custom callback upon destruction, which is set here:

ros2_control/hardware_interface/src/resource_manager.cpp

Lines 267 to 269 in 925f5f3

	return LoanedCommandInterface(
	resource_storage_->command_interface_map_.at(key),
	std::bind(&ResourceManager::release_command_interface, this, key));