add xarray layer

[rabernat]

It's great to see so much progress on xcape.

Part of our original vision was to have an xarray API, where you could pass xarray datasets / dataarrays and get them back.

Are you still interested in adding this type of interface?

[xebadir]

@rabernat how exactly would this work - I guess I'm not familiar with this style of implementation - as in you'd call the function on an xarray dataset/dataarray and then it would return the original array + the desired calculated variable rather than running it via a call of the core functions?