Avoid stations with negative upstream area in HydroGraph/position.

GraphHydro.py

@@ -144,7 +144,9 @@ class HydroGraph :
     def position(self, modelgrid, locations) :
         npt, nbas = self.routing_fetch.shape
         if len(locations.lid) > 0 :
-            for i,lid in enumerate(locations.lid) :
+          for i,lid in enumerate(locations.lid) :
+            # Some stations have a negative upstream area (because undefined)
+            if locations.upstream[i] > 0:
                 dist = RPP.distances(np.array(modelgrid.coordll)[:,0], np.array(modelgrid.coordll)[:,1], \
                                     locations.lon[i], locations.lat[i])
                 # Calculation in km