This signature is very confusing. `_peak` takes an `axis` argument that it doesn't use? What are `coords` and `dims`? All the other aggregators work on one dimension at a time. I would have thought it would be best to follow that pattern, so pass an array, an axis to operate on, and a set of axis values corresponding to the axis to be worked on (I assume these are needed to define the spline)?

Can you clarify this please?