Support median

[dcherian]

For convenience, it'd be nice to support medians even if it is relatively slow.

[dcherian]

https://github.com/xarray-contrib/flox/blob/627bf2b6818d8dd04051b13093a0d9c4f4a1df70/flox/aggregate_flox.py#L48

Here's a vectorized way to do quantiles if you're interested :)