fix: error out if shortest_time_between_assimilations <=0

shortest_time_between_assimilations is get_model_time_step

fixes #535

assimilation_code/modules/assimilation/obs_model_mod.f90

@@ -157,6 +157,9 @@ subroutine move_ahead(ens_handle, ens_size, seq, last_key_used, window_time, &
 
 ! Compute the model time step and center a window around the closest time
 delta_time = get_model_time_step()
+if (delta_time <= set_time(0,0)) then
+   call error_handler(E_ERR, 'move_ahead', 'shortest_time_between_assimilations must be > 0', source)
+endif
 
 ! print out current window, if requested
 if (print_trace_details > 0) then