tuning_tower: add is_active() method

Signed-off-by: Troy Jacobson <troy.d.jacobson@gmail.com>
Co-authored-by: Franklyn Tackitt <git@frank.af>

klippy/extras/tuning_tower.py

@@ -99,6 +99,8 @@ class TuningTower:
         self.gcode.respond_info("Ending tuning test mode")
         self.gcode_move.set_move_transform(self.normal_transform, force=True)
         self.normal_transform = None
+    def is_active(self):
+        return self.normal_transform is not None
 
 def load_config(config):
     return TuningTower(config)