Make save folders year first by default.

[colleenjg]

Changed default date formatting of utils.save_figure() to be year first ("%y_%m_%d") instead of day first ("%d_%m_%y").
Day first can be used by called save_figure() with year_first=False

[colleenjg]

Ok, following a quick discussion, I removed the option to set year_first=False and squashed the commit.

Note: Since this change changes folder names, it could transitionally cause some annoyance for users whose other code or directory structure expects "%d_%m_%y", when they start to get "%y_%m_%d" folders.

However, as discussed, the year first approach is probably better overall as it makes sorting by name chronological.

[TomGeorge1234]

the QOL update we never knew we needed. Thanks Colleen!