Add support for network and cooling fan

Add support for a network display, and for a coolin fan.  Note that at
the moment we support one and only one coolin fan, along with the CPU
fan.

monitor/fanspeed.py

@@ -0,0 +1,46 @@
+import RPi.GPIO as GPIO
+import time
+
+WAIT_TIME = 1
+
+class GetCaseFanSpeed:
+    TACH = 18
+    PULSE = 2
+    def __init__( self, tachPin = TACH):
+        self._tachPin = tachPin
+        self._rpm = 0
+        self._t   = time.time()
+        self.GPIO = GPIO
+        self.GPIO.setmode( self.GPIO.BCM )
+        self.GPIO.setwarnings( False )
+        self.GPIO.setup( self._tachPin, self.GPIO.IN, pull_up_down=self.GPIO.PUD_UP )
+        self.GPIO.add_event_detect( self._tachPin, self.GPIO.FALLING, self._calcRPM )
+        
+    def __del__( self ):
+        self.GPIO.cleanup()
+        
+    def _calcRPM( self, n ):
+        dt = time.time() - self._t
+        if dt < 0.005: return # Reject spuriously short pulses
+        
+        freq = 1 / dt
+        self._rpm = (freq / GetCaseFanSpeed.PULSE) * 60
+        self._t = time.time()
+    
+    @property
+    def RPM( self ):
+        return self._rpm
+    
+if __name__ == "__main__":
+    
+    fanSpeed = GetCaseFanSpeed()
+
+    try:
+        for i in range( 10 ):
+            print( f"{fanSpeed.RPM:.0f} RPM" )
+            time.sleep( 2 )
+        
+    except KeyboardInterrupt:
+        GPIO.cleanup()
+    
+