Postgis search fix

[SpacemanPaul]

Reason for this pull request

Queries against numeric search fields in the postgis driver were broken. The SQL queries were not being generated correctly AND the actual search index was not being populated correctly.

Proposed changes

• Fix SQL query generation for numeric searches

• Fix numeric search index population

• Add tests to ensure that similar issues are identified earlier in future.

• Tests added / passed

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

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08 🎉

Comparison is base (c7dae08) 85.10% compared to head (144252f) 85.18%.

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1475      +/-   ##
===============================================
+ Coverage        85.10%   85.18%   +0.08%     
===============================================
  Files              130      130              
  Lines            14348    14352       +4     
===============================================
+ Hits             12211    12226      +15     
+ Misses            2137     2126      -11     

Impacted Files	Coverage Δ
datacube/drivers/postgis/_api.py	87.34% <100.00%> (+0.07%)	⬆️
datacube/drivers/postgis/_fields.py	91.04% <100.00%> (+0.37%)	⬆️
datacube/index/postgis/_datasets.py	75.12% <100.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

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[omad]

lgtm