extract no data flag value and mark as numpy nan

stable_night_lights/nightlights_file_extraction.py

@@ -37,6 +37,8 @@ def read_proxydata(filename, dummy=DEBUG and USE_DUMMY_STABLE_NIGHT_LIGHTS_DATA)
 
     # construct data array and lonvec, latvec
     data = rst.bands[0].data()
+    nodata_value = rst.bands[0].nodata_value
+    data[data == nodata_value] = np.nan
     lonvec = np.linspace(xmin, xmax, cols)
     latvec = np.linspace(ymin, ymax, rows)