Make simpler API for using resampling infrastructure to do general gropuby

[wesm]

Using the TimeGrouper is both undocumented and not necessarily intended for average users. A cleaner, simpler API would be nice cc @dalejung related to bug in #2300
related to #3794

[dalejung]

On first glance, changing resample(how=None) to return the GroupBy would work. Or just replicating the function without the how.

More generally, I'd say the resample stuff should be split up into the datetime64 binning and the bin grouping. That would allow exposing a generalized bingroupby that could take in edges, edges/labels, bool edge arrays.

That's it from the top of my head but I'm about to dive into some funky groupby stuff with Panels. So I'll be back

[jreback]

maybe let's leave this open in order to document TimeGrouper in groupby.rst?

[cpcloud]

yeah i removed my comment for that reason

[jreback]

@dalejung want to do a PR for this?

[dalejung]

@jreback When I find time I'll gather some docs. I think I have some random notebooks I made to answer groupby/binning SO questions on nbviewer somewhere.

Though I'm not sure this addresses the issue. IIRC, the point was that TimeGrouper shouldn't be needed. Also, a GroupBy returned from TimeGrouper doesn't necessarily act exactly the same as a regular GroupBy. I'll have to poke around the internals again to remember.

[jreback]

closed by #6516