pid_calibrate: Update the heater's target temperature during calibration

Update the target temperature during the pid calibration.  This gives
additional feedback to the user and it makes it less likely that a
verify_heater error will be raised during calibration.

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

klippy/heater.py

@@ -105,6 +105,10 @@ class Heater:
             self.control = control
             self.target_temp = 0.
         return old_control
+    def alter_target(self, target_temp):
+        if target_temp:
+            target_temp = max(self.min_temp, min(self.max_temp, target_temp))
+        self.target_temp = target_temp
     def stats(self, eventtime):
         with self.lock:
             target_temp = self.target_temp