Incorporate sign_dt into next output to accomodate backwards time stepping

Author: fluidnumericsJoe

src/parcels/_core/particleset.py

@@ -422,11 +422,11 @@ def execute(
             pbar.set_description("Integration time: " + str(start_time))
 
         next_output = None
-        if output_file :
+        if output_file:
             # Write the initial condition
             output_file.write(self, start_time)
-            # Increment the next_output 
-            next_output = start_time + outputdt
+            # Increment the next_output
+            next_output = start_time + outputdt * sign_dt
 
         time = start_time
         while sign_dt * (time - end_time) < 0: