bugfixes for GPS and updated demo project

spatz/sensors/gps/gps.py

@@ -1,3 +1,5 @@
+import numpy as np
+
 from typing import Any, List
 from numpy.linalg import norm
 
@@ -27,7 +29,7 @@ class GPS(Sensor):
 
         # TODO: At which speed do we assume that GPS becomes unreliable?
         if vel / self._dataset.get_mach_number() > 1:
-            return NA
+            return np.array([0, 0, 0])
         
         x = self._dataset.fetch_values(['latitude', 'longitude', 'altitude'])
         x = self._sensor_specific_effects(x)