Add SNR property according to datasheet

Author: AdamLeyshon

adafruit_rfm9x.py

@@ -497,6 +497,16 @@ def rssi(self):
             raw_rssi -= 164
         return raw_rssi
 
+    @property
+    def snr(self):
+        """The SNR (in dB) of the last received message."""
+        # Read SNR 0x19 register and convert to value using formula in datasheet.
+        # SNR(dB) = PacketSnr [twos complement] / 4
+        snr_byte = self._read_u8(_RH_RF95_REG_19_PKT_SNR_VALUE)
+        if snr_byte > 127:
+            snr_byte = (256-snr_byte) * -1
+        return snr_byte/4
+
     @property
     def signal_bandwidth(self):
         """The signal bandwidth used by the radio (try setting to a higher