Add torque control to urscript

[domire8]

The PR adds MODE_TORQUE to the supported control modes and extends the external control urscript with a torque thread that applies the latest cmd_tau as torque command.

torque_command also has an optional parameter friction_comp that might be worth parametrizing too, but I'm not sure what effect that had in the first place so I went hardcoded for an MVP.

Let me know what you think about this and what might still be missing.

[codecov]

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##             torque_control     #325   +/-   ##
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[urfeex]

First of all, thank you very much for contributing this!

I mainly have two points at this stage:

• In my opinion it would be better to use torque instead of tau for readability. I added my perspective inside the first occurrence. Would you agree with that?
• Second, we've added an interface to set the friction compensation parameter on and off in #323, so we can use that in the torque_command call directly.

[domire8]

Second, we've added an interface to set the friction compensation parameter on and off in #323, so we can use that in the torque_command call directly.

Wonderful, that's neat!

[urfeex]

Thanks again for implementing and contributing this!