add "Not" operator

[Ariana-B]

Reason for this pull request

See #1560

Proposed changes

• Query terms are "OR"ed if provided as a list of values; there is no inbuilt type that could similarly be used to infer a "NOT", so create a custom Not type similarly to Range

• Create generic NotExpression similarly to OrExpression

• Closes "NOT" operator for ODC queries #1560

• Tests added / passed

• Fully documented, including docs/about/whats_new.rst for all changes

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📚 Documentation preview 📚: https://datacube-core--1563.org.readthedocs.build/en/1563/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.78%. Comparing base (bfb8417) to head (0b550ad).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1563   +/-   ##
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  Coverage    91.77%   91.78%           
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  Files          132      132           
  Lines        14631    14641   +10     
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+ Hits         13428    13438   +10     
  Misses        1203     1203           

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[SpacemanPaul]

Mostly looks great, but as noted I don't like the name NotType - can we just call it Not?

[SpacemanPaul]

Conditional approval.