Added string support and setylim for stat_smooth

[MxMarx]

I tested it and I don't think this breaks anything.

Two changes:

I copied setylim from stat_summary into stat_smooth so it works for both

I added string support to unique_and_sort.m so you can have string variables as well as categorical and cells. Might be able to add this to unique_non_nan.m but I don't know enough about it.