Add bin_by argument to compute_statistic

[astrofrog]

It would be good to add a bin_by argument to compute_statistic, as described in https://gist.github.com/astrofrog/23bd8eefbfb00928effe146b96b8fe18 - this would be similar to what vaex does, and allows more powerful statistical aggregated visualizations.

@maartenbreddels - does the API in the above gist seem reasonable? Would it be easy to pass that option to vaex behind the scenes?

With this in place, we could actually deprecate compute_histogram.

[maartenbreddels]

@maartenbreddels - does the API in the above gist seem reasonable? Would it be easy to pass that option to vaex behind the scenes?

Yes, that seems like a good idea.

With this in place, we could actually deprecate compute_histogram.

indeed.