Prepare for xarray 0.11

[PeterDSteinberg]

Just saw this warning come up:

FutureWarning: iteration over an xarray.Dataset will change in xarray v0.11 to only include data variables, not coordinates. Iterate over the Dataset.variables property instead to preserve existing behavior in a forwards compatible manner.

Currently I'm on the latest xarray (0.10 - https://github.com/pydata/xarray/releases).

We'll need to change the iteration over xarray.Dataset-like objects. This relates to:

• Elm
• Earthio
• Xarray_filters

[PeterDSteinberg]

Hi @gbrener - assigned this to you. It can be addressed before 0.11 xarray release or temporary fix in environment specs in each of the 3 repos, as needed, if we need to defer the work and just stick to xarray=0.10 for a bit. Let's do that the temporary fix plan, if there's any risk of this destabilizing the repos before Dec 18.