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/delta.py

@@ -88,7 +88,7 @@ class DeltaKinematics:
                         math.sqrt(self.very_slow_xy2)))
         self.axes_min = toolhead.Coord(-max_xy, -max_xy, self.min_z, 0.)
         self.axes_max = toolhead.Coord(max_xy, max_xy, self.max_z, 0.)
-        self.set_position([0., 0., 0.], ())
+        self.set_position([0., 0., 0.], "")
     def get_steppers(self):
         return [s for rail in self.rails for s in rail.get_steppers()]
     def _actuator_to_cartesian(self, spos):
@@ -101,7 +101,7 @@ class DeltaKinematics:
         for rail in self.rails:
             rail.set_position(newpos)
         self.limit_xy2 = -1.
-        if tuple(homing_axes) == (0, 1, 2):
+        if homing_axes == "xyz":
             self.need_home = False
     def clear_homing_state(self, axes):
         # Clearing homing state for each axis individually is not implemented