klippy: Add ConfigWrapper.getchoice method

Add helper function that ensures a config option is one of several
choices.  This helps ensure that a proper error is raised if an
invalid choice is made.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/toolhead.py

@@ -161,8 +161,7 @@ class ToolHead:
         self.extruder = printer.objects.get('extruder')
         kintypes = {'cartesian': cartesian.CartKinematics,
                     'delta': delta.DeltaKinematics}
-        kin = config.get('kinematics', 'cartesian')
-        self.kin = kintypes[kin](printer, config)
+        self.kin = config.getchoice('kinematics', kintypes)(printer, config)
         self.max_speed, self.max_accel = self.kin.get_max_speed()
         self.junction_deviation = config.getfloat('junction_deviation', 0.02)
         self.move_queue = MoveQueue()