Use context manager for xarray dataset file opening

[spjuhel]

Changes proposed in this PR:
Replace all occurrence of data = xr.open_dataset(filename) with with xr.open_dataset(filename) as data: to ensure filestream closing. See #920 on why this can cause problems.

Using data = xr.open_dataset(filename) assumes a data.close() afterwards to correctly close the filestream. This is not done in all function. The pythonic approach is to use a context manager with ... as ... : which handle the closing.

See also: pydata/xarray#2887

This PR fixes #920

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[spjuhel]

Should we also mention this in the programming guidelines?

[chahank]

Yes!

[emanuel-schmid]

Very valuable contribution. 🙌

I'd just change the changelog

1. this is a fix not a change
2. the log should tell what has been done, but not how. If somebody is interested they can click on the PR link.