fix altitude for telemetry, instead of using zero

src/helpers/SensorManager.h

@@ -11,8 +11,9 @@
 class SensorManager {
 public:
   double node_lat, node_lon;  // modify these, if you want to affect Advert location
+  double node_altitude;       // altitude in meters
 
-  SensorManager() { node_lat = 0; node_lon = 0; }
+  SensorManager() { node_lat = 0; node_lon = 0; node_altitude = 0; }
   virtual bool begin() { return false; }
   virtual bool querySensors(uint8_t requester_permissions, CayenneLPP& telemetry) { return false; }
   virtual void loop() { }

src/helpers/sensors/LocationProvider.h

@@ -8,6 +8,7 @@ class LocationProvider {
 public:
     virtual long getLatitude() = 0;
     virtual long getLongitude() = 0;
+    virtual long getAltitude() = 0;
     virtual bool isValid() = 0;
     virtual long getTimestamp() = 0;
     virtual void reset();

src/helpers/sensors/MicroNMEALocationProvider.h

@@ -61,6 +61,11 @@ public :
 
     long getLatitude() override { return nmea.getLatitude(); }
     long getLongitude() override { return nmea.getLongitude(); }
+    long getAltitude() override { 
+        long alt = 0;
+        nmea.getAltitude(alt);
+        return alt;
+    }
     bool isValid() override { return nmea.isValid(); }
 
     long getTimestamp() override {