Fix writing of 'dt' when 'stopnow' is used

When the 'stopnow' file is created to immediately stop a run,
`t_params.dt_before_output[]` was previously not set, because the
`adaptive_timestep_update_t_params!()` function does not know that there
is going to be an output triggered by 'stopnow', so 'dt' was written
incorrectly. Fix this by setting
`t_params.dt_before_output[] = t_params.dt[]` when 'stopnow' is found.

moment_kinetics/src/time_advance.jl

@@ -1921,6 +1921,7 @@ function  time_advance!(pdf, scratch, scratch_implicit, scratch_electron, t_para
                 # Stop cleanly if a file called 'stop' was created
                 println("Found 'stopnow' file $(t_params.stopfile * "now"), aborting run")
                 finish_now = true
+                t_params.dt_before_output[] = t_params.dt[]
             end
 
             if t_params.adaptive && !t_params.write_after_fixed_step_count