GazeboYarpControlBoardDriver getPidErrors seems to not work properly for torque PID

[Nicogene]

In robotology/robometry#137 @isorrentino experienced the same issue of #555, getPidErrors seems to return false.

Moreover, getPidErrors seem to return always the position error, independently from the vocab passed to the fuction

gazebo-yarp-plugins/plugins/controlboard/src/ControlBoardDriverPidControl.cpp

Lines 120 to 136 in 0d18d2e

	bool GazeboYarpControlBoardDriver::getPidError(const PidControlTypeEnum& pidtype, int j, double *err)
	{
	PIDMap::iterator pidType = m_pids.find(pidtype);
	if (pidType == m_pids.end()) {
	yWarning() << "Could not find PID for type " << pidtype;
	return false;
	}
	if (j < 0 || static_cast<size_t>(j) >= pidType->second.size()) {
	yWarning() << "Joint index must be between 0 and " << pidType->second.size();
	return false;
	}
	*err = (m_positions[j] - m_zeroPosition[j]) - m_motReferencePositions[j];
	return true;
	}