Improve yarpmotorgui open speed

[S-Dafarra]

During the XPrize finals, we noticed that the yarpmotorgui can take some time to start.

I check the code and I found that there are some checks on whether there are other yarpmotogui opened. See for example

yarp/src/yarpmotorgui/partitem.cpp

Line 86 in 76fe34d

Contact adr=Network::queryName(nameToCheck.toLatin1().data());

My guess is that this call can be costly in case of a delayed network.

Moreover, there was a useless call to get each joint position PID:

yarp/src/yarpmotorgui/partitem.cpp

Lines 230 to 232 in 76fe34d

	Pid myPid(0,0,0,0,0,0);
	yarp::os::SystemClock::delaySystem(0.005);
	m_iPid->getPid(VOCAB_PIDTYPE_POSITION, k, &myPid);

Finally, there are some delays to make sure that the reading of the joint values is valid:

yarp/src/yarpmotorgui/partitem.cpp

Lines 184 to 194 in 76fe34d

	bool ret = false;
	SystemClock::delaySystem(0.050);
	do {
	ret = m_iencs->getEncoders(m_positions);
	if (!ret) {
	yError("%s iencs->getEncoders() failed, retrying...\n", partName.toLatin1().data());
	SystemClock::delaySystem(0.050);
	}
	} while (!ret);
	yInfo("%s iencs->getEncoders() ok!\n", partName.toLatin1().data());

Hence, with this PR:

• I have added a dummy port such that the check to avoid conflicts is done only once, instead of multiple times per part.
• I have removed the useless PID request.
• I have parallelized the opening of the different parts using std::async

I have measured the initialization time before and after when opening yarpmotorgui with the robot on Gazebo in my PC (so all local communication), and the improvement was more than half a second.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would update the release notes by adding a file in doc/release/<target_branch>, based on your changes.

[S-Dafarra]

In agreement with @randaz81, I will try to run one test also on Windows and eventually Mac to make sure that the new multithreaded behavior is not causing issues.