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/toolhead.py

@@ -457,7 +457,7 @@ class ToolHead:
     # Movement commands
     def get_position(self):
         return list(self.commanded_pos)
-    def set_position(self, newpos, homing_axes=()):
+    def set_position(self, newpos, homing_axes=""):
         self.flush_step_generation()
         ffi_main, ffi_lib = chelper.get_ffi()
         ffi_lib.trapq_set_position(self.trapq, self.print_time,