Null template index driver

[SpacemanPaul]

Proposed changes

Implement a "null" index driver - an always empty index

Reason for this pull request

Mostly this a validation that alternate index drivers can be defined and tested, and a template for implementing future index drivers. It may also be convenient for ODC use cases that do not require an index.

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

[codecov]

Codecov Report

Merging #1236 (8711689) into develop (38a314a) will increase coverage by 1.14%.
The diff coverage is 97.25%.

@@             Coverage Diff             @@
##           develop    #1236      +/-   ##
===========================================
+ Coverage    92.71%   93.86%   +1.14%     
===========================================
  Files          103      109       +6     
  Lines        10598    10757     +159     
===========================================
+ Hits          9826    10097     +271     
+ Misses         772      660     -112     

Impacted Files	Coverage Δ
datacube/index/postgres/_products.py	94.01% <ø> (+0.07%)	⬆️
datacube/index/null/_users.py	90.00% <90.00%> (ø)
datacube/index/abstract.py	95.73% <93.75%> (+0.83%)	⬆️
datacube/index/null/index.py	94.11% <94.11%> (ø)
datacube/index/null/__init__.py	100.00% <100.00%> (ø)
datacube/index/null/_datasets.py	100.00% <100.00%> (ø)
datacube/index/null/_metadata_types.py	100.00% <100.00%> (ø)
datacube/index/null/_products.py	100.00% <100.00%> (ø)
datacube/index/postgres/_metadata_types.py	93.97% <100.00%> (-0.28%)	⬇️
datacube/index/postgres/index.py	100.00% <100.00%> (ø)
... and 5 more

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[alexgleith]

Seems reasonable to me. Love that you're tacking all this!