New feature: folded SFS
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Thanks to work by apragsdale, demographic history can be inferred from the folded SFS using reference-polarized data instead of ancestral-state-polarized data. See the new `folded` option for [`kSFS.infer_history()`](https://harrispopgen.github.io/mushi/mushi.html#mushi.kSFS.infer_history) and [`kSFS.plot_total`](https://harrispopgen.github.io/mushi/mushi.html#mushi.kSFS.plot_total)