Support for combined robot hardware

[asierfernandez]

We propose the minimum set of changes to use the driver together with combined_robot_hw.

In our setup we use a haptic device with an UR robot using the combined_robot_hw and it works fine. This change doesn't break anything, and even if this is not the final solution, at least someone could use it as we do.

For example, we see some limitations to how the combined_robot_hw together with the driver handles read operations. If I'm correct read is a blocking operation, and the combined_robot_hw executes this operation for all hardwares consecutively. Even if I haven't tested yet, if we combine multiple arms together with with some extra hardware it won't work properly.

There are many things to discuss about combined_robot_hw, and it would be good to start discussing about it, if there is a real interest of course.

[gavanderhoorn]

Besides logistical issues (such as with the libraries) it would also seem the way a combined robot hw interface is orchestrated would be something to think about (as mentioned by @asierfernandez).

In the current implementation the robot controller mostly dictates control flow. I'm not sure how suitable that is when it is part of a larger composite.

[fmauch]

Besides logistical issues (such as with the libraries) it would also seem the way a combined robot hw interface is orchestrated would be something to think about (as mentioned by @asierfernandez).

In the current implementation the robot controller mostly dictates control flow. I'm not sure how suitable that is when it is part of a larger composite.

Yes, we could definitely discuss this (Separately from this PR?)

We deliberately decided to let the robot dictate the timing to make real-time operation a bit easier.

@asierfernandez @gavanderhoorn Is it fine with you to merge this PR without this being tackled for the moment and we can discuss this in a separate issue?

[fmauch]

Seems fine to me

[gavanderhoorn]

If the currently supported use-cases are still supported, we could consider merging this in as an initial effort towards CombinedRobotHW compatibility.

It may be nice though to explicitly document the fact that the robot controller dictates control flow for this plugin and that all reads() are blocking.

[asierfernandez]

It's fine for me to merge it and creating a different issue #38 for discussion.
The goal of opening the issue was partially to create a discussion about this topic.

[fmauch]

@asierfernandez

#86 implements a non-blocking read. Would you agree on the implementation there?