Make sure graphic notebooks start from scratch or check downloaded JSON

[lee1043]

Comment from @acordonez (https://github.com/PCMDI/pcmdi_metrics/pull/843#pullrequestreview-962641185)

"The notebooks worked for me. One thing I noticed is that I had some old files in my portrait_plot/json_files that hadn't downloaded correctly in a past test (contained 404 errors). This caused a failure in the portrait plot notebook until I removed the existing json_files folder and started from scratch. Would the notebook still give the correct results if users had old versions of the same data in json_files? Should the json_files be cleaned before running each time?"