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/extras/manual_stepper.py

@@ -109,7 +109,7 @@ class ManualStepper:
         self.sync_print_time()
     def get_position(self):
         return [self.rail.get_commanded_position(), 0., 0., 0.]
-    def set_position(self, newpos, homing_axes=()):
+    def set_position(self, newpos, homing_axes=""):
         self.do_set_position(newpos[0])
     def get_last_move_time(self):
         self.sync_print_time()