Show fan speed according to max_power and off_below (#543)

fixes #444

ks_includes/printer.py

@@ -291,6 +291,22 @@ class Printer:
             return self.devices[dev][stat]
         return None
 
+    def get_fan_speed(self, fan="fan", speed=None):
+        if fan not in self.config:
+            logging.debug("Error getting %s config", fan)
+            return speed if speed is not None else 0
+        if speed is None and "speed" in self.data[fan]:
+            speed = self.data[fan]["speed"]
+        if 'max_power' in self.config[fan]:
+            max_power = float(self.config[fan]['max_power'])
+            if max_power > 0:
+                speed = speed / max_power
+        if 'off_below' in self.config[fan]:
+            off_below = float(self.config[fan]['off_below'])
+            if speed < off_below:
+                speed = 0
+        return speed
+
     def get_extruder_count(self):
         return self.extrudercount