stepper: Have caller calculate max jerk velocity

Allow the owner of the stepper object to cacluate the maximum step
jerk velocity.  This is used to ensure there is no communication error
between mcu and host.

Disable checking of jerk velocity for extruder stepper motors.

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

klippy/cartesian.py

@@ -15,6 +15,8 @@ class CartKinematics:
                          for n in steppers]
         self.stepper_pos = [0, 0, 0]
     def build_config(self):
+        for stepper in self.steppers[:2]:
+            stepper.set_max_jerk(0.005 * stepper.max_accel) # XXX
         for stepper in self.steppers:
             stepper.build_config()
     def set_position(self, newpos):