Bug fix: correct sign of variation of Rossby number and radius

Bug fix: correct sign of variation of Rossby number and radius. We
have to be consistent with the computation of `delta_ro_mean` and
`delta_r_mean`. `delta_ro_mean` and `delta_r_mean` were computed by
subtracting origin from tail.

Trajectories/cost_function.py

@@ -245,13 +245,13 @@ for edge in g.edges():
 
     # Rossby numbers:
     if first_av_ros[target_node] and last_av_ros[source_node]:
-        Delta_Ro = last_av_ros[source_node] - first_av_ros[target_node]
+        Delta_Ro = first_av_ros[target_node] - last_av_ros[source_node]
     else:
         # At least one of the rossbies is invalid.
         Delta_Ro = 0
 
     # R_Vmax 1 and 2 already exist, just get the delta
-    Delta_R_Vmax = last_av_rad[source_node] - first_av_rad[target_node]
+    Delta_R_Vmax = first_av_rad[target_node] - last_av_rad[source_node]
 
     # Calculate the cost and assign to the edge:
     g.ep.cost_function[edge] = math.sqrt(