Expose station_alt to observations API

For an observation we already save the TLE and return the station's
lattitude and longitude.  To accurately reconstruct the geometry of the
situation, we also need to return the station's altitude.

Closes #486

network/api/serializers.py

@@ -15,16 +15,18 @@ class ObservationSerializer(serializers.ModelSerializer):
     station_name = serializers.SerializerMethodField()
     station_lat = serializers.SerializerMethodField()
     station_lng = serializers.SerializerMethodField()
+    station_alt = serializers.SerializerMethodField()
     demoddata = DemodDataSerializer(many=True)
 
     class Meta:
         model = Observation
         fields = ('id', 'start', 'end', 'ground_station', 'transmitter', 'norad_cat_id',
                   'payload', 'waterfall', 'demoddata', 'station_name', 'station_lat',
-                  'station_lng', 'vetted_status', 'client_version', 'client_metadata')
+                  'station_lng', 'station_alt', 'vetted_status', 'client_version',
+                  'client_metadata')
         read_only_fields = ['id', 'start', 'end', 'observation', 'ground_station',
                             'transmitter', 'norad_cat_id', 'station_name',
-                            'station_lat', 'station_lng']
+                            'station_lat', 'station_lng', 'station_alt']
 
     def update(self, instance, validated_data):
         validated_data.pop('demoddata')
@@ -58,6 +60,12 @@ class ObservationSerializer(serializers.ModelSerializer):
         except AttributeError:
             return None
 
+    def get_station_alt(self, obj):
+        try:
+            return obj.ground_station.alt
+        except AttributeError:
+            return None
+
 
 class StationSerializer(serializers.ModelSerializer):
     antenna = serializers.SerializerMethodField()