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/cartesian.py

@@ -65,7 +65,8 @@ class CartKinematics:
     def set_position(self, newpos, homing_axes):
         for i, rail in enumerate(self.rails):
             rail.set_position(newpos)
-        for axis in homing_axes:
+        for axis_name in homing_axes:
+            axis = "xyz".index(axis_name)
             if self.dc_module and axis == self.dc_module.axis:
                 rail = self.dc_module.get_primary_rail().get_rail()
             else: