add composite_analysis and xr_ttests

[aaronspring]

create composite from timeseries and field.
check whether composite is different from mean with ttest

composite_analysis(
    pCO2, enso, plot=True, threshold=1, robust=True)

[bradyrx]

@aaronspring I need to actually review these two PRs because they look very useful. Been so focused on climpred I forgot about this stuff.

[aaronspring]

no worries. Would be nice to chat about those. It has been ages. Used it for some supplementary plot in my paper draft

[aaronspring]

Thanks for the revision and adapting to the updated code. Can this now go into master? LGTM

[bradyrx]

Those were a few preliminary edits. Checking that this works with CESM output and might refactor slightly, then will merge.

[bradyrx]

Will request a review once I'm done with this. Have a few more ideas.

[bradyrx]

@aaronspring , give this a look and let me know what you think. This is ready to merge unless we want to clean up the warnings catch. Also, ttest is sort of slow, but there's nothing we can do about that I think.

[aaronspring]

general comment: I am unsure whether parallelized is really a good idea.
Tip on http://xarray.pydata.org/en/stable/dask.html suggests:

For the majority of NumPy functions that are already wrapped by Dask, it’s usually a better idea to use the pre-existing dask.array function, by using either a pre-existing xarray methods or apply_ufunc() with dask='allowed'. Dask can often have a more efficient implementation that makes use of the specialized structure of a problem, unlike the generic speedups offered by dask='parallelized'.

[aaronspring]

I will have to investigate this for xskillscore in detail. I think it doesnt really change much, but I cannot prove it yet

[bradyrx]

I can try running this on CESM data with parallelized and allowed and see timing differences. The issue is a dask array can't be put through the function because earlier lines of code (before the t-test) require it to be loaded into memory.

[aaronspring]

the warnings catch is ok to me. I cannot review as the PR is mine.

[bradyrx]

@aaronspring, I'll merge this now then. You can check on the parallelized vs. allowed and let me know what you find. Thanks for this... I really like this function. Will make composites so much easier! Great work! Looking forward to developing this package a bit now with our experience from climpred.