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/ZsensorDHT.ino

@@ -63,7 +63,9 @@ void MeasureTempAndHum() {
         Log.trace(F("Same hum don't send it" CR));
       }
       if (t != persistedt || dht_always) {
-        DHTdata.set("temp", (float)t);
+        DHTdata.set("temp", (float)t); // remove for 0.9.6 release
+        DHTdata.set("tempc", (float)t);
+        DHTdata.set("tempf", dht.convertCtoF(t));
       } else {
         Log.trace(F("Same temp don't send it" CR));
       }