Stats func accessor 59

[lukegre]

Description

Added xarray accessors for the stats functions. Added accessor as stats to xr.DataArray (and not xr.Dataset).
Includes: corr, linear_slope, linregress, polyfit, rm_poly, rm_trend

Did not change the API of these functions as I originally intended - I didn't realise how extensive the tests were and this would all have to be changed 😵

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Manually. Not a complicated implementation of xarray accessors.

Checklist (while developing)

• I have added docstrings to all new functions.
• I have updated the sphinx documentation, if necessary.

[lukegre]

Travis tests are failing on stats.rm_poly for the dask arrays when called with x and y. When y is forced to load(), the tests pass. Not sure why the docs build is failing.
I have not made any changes to stats functions.

[aaronspring]

please raise a PR on the failing stats functions. new versions of xr and/or dask likely the reason

[bradyrx]

please raise a PR on the failing stats functions. new versions of xr and/or dask likely the reason

@aaronspring I feel like we recently dealt with this in climpred? There was some break due to the new versions and I can't remember what we had to change. I think you were working on it.

[bradyrx]

@luke-gregor, if you rebase this onto master the bug should be fixed and you should be good to go to proceed with this.

[bradyrx]

This will be fixed from #105. You can rebase once I merge it when the RTD tests pass and so on. Sorry this little implementation has been such a headache! But you found a few holes with recent upstream changes. :)

[bradyrx]

Okay @luke-gregor, another rebase onto master should do the trick here!

[lukegre]

All passed! I'll update the docs accordingly.

[bradyrx]

Thanks @luke-gregor! Sorry for the delay here. In the final stretch of writing, so haven't kept too close of an eye on Github. Merging now!