Export SimHWInterface for combined_robot_hw

[RobertWilbrandt]

It is surprisingly useful to have a purely kinematic simulation of a hardware interface in a combined_robot_hw. Using a custom adapter i already used it to allow for partially-simulated execution in a more complex combined_robot_hw setup, and it generally makes switching between real hardware and simulation in such projects much easier. I would however prefer this to be an upstream feature of ros_control_boilerplate (and am happy to provide a PR for it) for the following reasons:

• It would demonstrate how to properly write and export a hw interface using pluginlib
• Adding a launch file to rrbot_control could demonstrate how to use a combined_robot_hw
• It can be quite handy to use this as a placeholder when developing bigger combined_robot_hw setups

Unfortunately i don't think simply changing SimHWInterface (or GenericHWInterface) would be possible without quite some restructuring, as pluginlib requires a parameterless constructor. Thus i would create an extra adapter class deriving from hardware_interface::RobotHW which would hold a SimHWInterface as a member and delay it's construction until init(), simply delegating all relevant calls from there on.

The way i see it the required steps for this are:

• Create CombinedHWAdapter class
• Export interface correctly as ros_control_boilerplate/SimHWInterface
• Add node src to rrbot_control. Unfortunately running a combined_robot_hw from a GenericHWControlLoop doesn't seem too nice.
• Add sample launch files to rrbot_control
• Add short description to README.md

I'm happy to hear about opinions, otherwise i'd start implementing this in the next days.

[Tuebel]

I have just stumbled over this issue after opening PR #42 which is intended to bring support for combined_robot_hw.
Definitely agree that an adapter could be a good solution. More concerns and ideas can be found in the PR, I don't want to cross-post too much.