Standardize MQTTDiscovery Temp Keys [deprecate tem key] (#703)

* Detect when sensor isn't available or read fails

* Add temperature conversion macros

* Add 'tempc' & 'tempf' keys to published values

* Unique Values for HA Discovery

* Remove degree symbol from temp sensors

Co-authored-by: Jonathan Michael Wells <jonathan-wells@oist.jp>

main/ZgatewayWeatherStation.ino

@@ -105,7 +105,9 @@ void sendTemperatureData(byte id, float temperature, int humidity, byte battery_
     StaticJsonBuffer<JSON_MSG_CALC_BUFFER> jsonBuffer;
     JsonObject& RFdata = jsonBuffer.createObject();
     RFdata.set("sensor", id);
-    RFdata.set("temperature", temperature);
+    RFdata.set("temperature", temperature); // remove for 0.9.6 release
+    RFdata.set("tempc", temperature);
+    RFdata.set("tempf", wsdr.convertCtoF(temperature));
     RFdata.set("humidity", humidity);
     RFdata.set("battery", bitRead(battery_status, 0) == 0 ? "OK" : "Low");
     pub(subjectRFtoMQTT, RFdata);