ENSO portrait plot color code update in notebook

[lee1043]

Testing #1372 and document its outcome

[lee1043]

Hi @zhangshixuan1987, could you please take a look the following notebook to see the change is consistent to that you expected?

https://github.com/PCMDI/pcmdi_metrics/blob/71db16aac31cbb2f62d1f1ccb70875df0ecd510f/pcmdi_metrics/enso/lib/summary_plot_lib/enso_portrait_plot.ipynb

[zhangshixuan1987]

@lee1043 Hi Jiwoo, the results look consistent with what I expected. I wanted to ask whether the color scale looks good to you, or if you think we should adjust it to improve the visualization.