It looks to me like applying `_peak` to multiple coordinates behaves differently from other aggregators. It seems to operate on each coordinate *in turn* rather than all at once, making the results order-dependent. Given the coordinates are sorted it will always give consistent results (ie. `cube.collapsed(['lat', 'lon'], PEAK) == cube.collapsed(['lon', 'lat'], PEAK)`), but I'm not convinced they're particularly meaningful. If that's the case, perhaps PEAK should refuse to deal with multiple coordinates somehow...? (In which case the `dims` argument would be redundant.)