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/main.ino

@@ -168,6 +168,9 @@ WiFiClient eClient;
 EthernetClient eClient;
 #endif
 
+#define convertTemp_CtoF(c) ((c * 1.8) + 32)
+#define convertTemp_FtoC(f) ((f - 32) * 5 / 9)
+
 // client link to pubsub mqtt
 PubSubClient client(eClient);