DOC: show how to use to_xarray then use xarray to perform analysis on Panels

[jreback]

I think we need a section to address this, not really sure where should put it. Also want to link from the doc-string.

also prob need some examples in the doc-string.

xref #11972

[jreback]

cc @MaximilianR
@jorisvandenbossche

not strictly necessary for 0.18.0 but if you can do something would be great.

[max-sixty]

Yes, I have something for this, I will PR today or tomorrow

[jreback]

cc @MaximilianR

[max-sixty]

D'oh! I wrote this back in Feb but forgot to PR. Just done that here: pydata/xarray#832

[jorisvandenbossche]

@MaximilianR Would you like to add such a guide in the pandas docs as well? (not sure if it should be in both, but it feels like if we point users to xarray we should also give them some guidance)

[max-sixty]

@jorisvandenbossche yes I can do that. Any feedback on the original PR?

[shoyer]

Note that this now appears in the xarray docs: http://xarray.pydata.org/en/stable/pandas.html#transitioning-from-pandas-panel-to-xarray