Quantify dataset v2

Explanation of changes

This MR is intended to replace the dataset design MRs !212 (closed) (and !208 (closed)).

Includes:

• detailed specification that should be close to "final" for a quantify dataset v2
• auxiliary tools expected to be part of the code and are used to ensure the proposed specification is feasible.
  • BUT please let's not nitpick, yet, about e.g. naming of func./methods/classes/modules, modules location, etc.;
• sphinx extension proposal from !220 (merged) (don't nitpick about it in this MR please, use !220 (merged) for that)

Important feedback I am looking for:

• Do you agree with the dataset attributes?
• Do you agree with the attributes round-trip scheme? (expected to close #158 (closed) as well)
• Feedback on the specification contents, is it clear/enough?

To do:

• write supporting text in dataset example "notebooks".
• overall clean up.

Resources:

• Subclassing xarray by Pieter: #233 (comment 643437076)

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Should address at least part of #187 (closed)

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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 has 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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For reference, the issues workflow is described in the contribution guidelines.