scan move back motors to start pos should ignore calcAxis with Nan pos
This can happen with calc of calc and "one way CalcMotors" (i.e. calc=>reals can be computed but reals=>calc has no mathematical solution). In that case pseudo_pos=Nan means that the real-motors are not at the positions corresponding to a valid pseudo pos. In that case the scan should just ignore those axes and should not try to move them back after scan end (actually those axes did not move during the scan and where already Nan before scan starts).