Improve data updates and plot generation

[mmacferrin]

• Updated "update_data.py" to work with nsidc-0080 v2 data.
• Created a generate_plots_for_given_day.py script to allow plots to be generated for any past day in the time-series without making any changes to the underlying database. This is a bit different than "update_data.py", which downloads & updates the database and then generates new plots for whatever the "latest" day is only, but does not explicitly allow generation of plots for previous days. (This allowed me to go back and generate end-of-year plots for the last day of the previous melt season, for instance).

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[mfisher87]

Merging into a v2 branch to consolidate all our work in to one place which will be merged in to main.