Implement getCurrents

[traversaro]

We already knew about this (see

gz-sim-yarp-plugins/plugins/controlboard/src/ControlBoardDriver.cpp

Lines 1368 to 1377 in 6b25488

	bool ControlBoardDriver::getCurrent(int m, double* curr)
	{
	// TODO
	return true;
	}
	bool ControlBoardDriver::getCurrents(double* currs)
	{
	// TODO
	return true;
	}

), but I just wanted to open a dedicated issue as this would be useful for @carloscp3009 .

For the implementation, as now getTorque actually return the torque that the output shaft of the transmission is applying to the mechanical structures, it make sense that getCurrents instead returns the "motor torque", divided by the k_tau or similar constant.

[traversaro]

fyi @GiulioRomualdi