unstable_sort_by panics when given NaN

[Roughsketch]

There is a panic on the following line when one of the values is NaN:

delaunator-rs/src/lib.rs

Line 434 in 82ddbdb

dists.sort_unstable_by(|&(_, da), &(_, db)| da.partial_cmp(&db).unwrap());

You can get around this by implementing a default ordering with something like

dists.sort_unstable_by(|&(_, da), &(_, db)| da.partial_cmp(&db).unwrap_or(std::cmp::Ordering::Less));

Or by checking if a given argument contains a NaN in the first place if NaN breaks further work. I think having one of these and returning None is preferable to a panic inside the method. At the very least this behavior should be documented.

[mourner]

Fixed by #28