Dataset v2 implementation

Explanation of changes

This MR has quite a bit of changes, but for now I would appreciate mostly a 👍 on the user interface for the utilities that implement (at least partially) Quantify dataset functionality.

Namely, please have a look at:

• Xarray accessor (the recommended alternative to subclassing xarray).

And how it is used in the examples (just search in the browser for .qcore. on these pages and see the diff for the source-code of these pages):

• https://quantify-quantify-core--243.com.readthedocs.build/en/243/technical_notes/dataset_design/Quantify%20dataset%20-%20specification.py.html
• https://quantify-quantify-core--243.com.readthedocs.build/en/243/technical_notes/dataset_design/Quantify%20dataset%20-%20examples.py.html
• https://quantify-quantify-core--243.com.readthedocs.build/en/243/technical_notes/dataset_design/Quantify%20dataset%20-%20advanced%20examples.py.html

Let me know what you think.

If you have time you can comment on the rest of this MR as well.

Motivation of changes

Progress on %"v0.6 Dataset v2"

Closes #254

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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.
• CHANGELOG.rst and AUTHORS.rst have been updated (when applicable).
• 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 successfully (CI and readthedocs),
  • windows tests pass (manually triggered by maintainers before merging).

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