toolhead: Pass set_position() homing_axes parameter as a string

Use strings such as "xyz" to specify which axes are to be considered
homing during a set_position() call.  This makes the parameter a
little less cryptic.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/kinematics/winch.py

@@ -26,7 +26,7 @@ class WinchKinematics:
         acoords = list(zip(*self.anchors))
         self.axes_min = toolhead.Coord(*[min(a) for a in acoords], e=0.)
         self.axes_max = toolhead.Coord(*[max(a) for a in acoords], e=0.)
-        self.set_position([0., 0., 0.], ())
+        self.set_position([0., 0., 0.], "")
     def get_steppers(self):
         return list(self.steppers)
     def calc_position(self, stepper_positions):