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/extruder.py

@@ -14,6 +14,7 @@ class PrinterExtruder:
         self.stepper_pos = 0
     def build_config(self):
         self.heater.build_config()
+        self.stepper.set_max_jerk(9999999.9)
         self.stepper.build_config()
     def get_max_speed(self):
         return self.stepper.max_velocity, self.stepper.max_accel