bisector.center - #156

And the demo https://observablehq.com/d/8581f0e7e07ee62b

It’s clever how this uses the comparator for most of the work and only relies on the signed distance calculation at the end to choose between the two possible results, such that if an accessor is specified and the accessor doesn’t return a number, the result is NaN and this is equivalent to bisector.left.

There’s a bug here with arrays of size zero and one. I’ll add a fix.

Oh, and also this needs to the support the optional lo and hi arguments.

I found it surprising that bisector.center returns values rather than indexes, unlike bisector.left and bisector.right. So I’ve also rewritten the code to return indexes. And I noticed that it wasn’t consistent with bisector.left in the ordinal case, so I’ve also added some more tests.