Implemented numpy.bool_ patch for xarray 0.17.0

Explanation of changes

Solves a bug in xarray 0.17.0 that is blocking us from saving a processed_dataset in the Analysis classes.

This is a temporary solution to #161 (closed)

Motivation of changes

Even though our pull request to fix issue this in the xarray was accepted they have introduced breaking changes so we should only adapt our code when a new release of xarray is out, in the meantime we just use this patch.

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Merge checklist

See also merge request guidelines

• Merge request has been reviewed and approved by a project maintainer.
• Merge request contains a clear description of the proposed changes and the issue it addresses.
• Merge request made onto appropriate branch (develop for most MRs).
• New code is fully tested.
• New code is documented and docstrings use numpydoc format.
• CI pipelines pass
  • black code-formatting passes (gitlab-ci),
  • test suite passes (gitlab-ci),
  • no degradation in code-coverage (codacy),
  • no (serious) new pylint code quality issues introduced (codacy),
  • documentation builds succesfully (readthedocs).