Spatial index api

[SpacemanPaul]

Reason for this pull request

Formalise the Spatial index API and expose in CLI.

Proposed changes

• API method for dropping/removing an existing spatial index.

• Formalise spatial index API at the index/abstract.py level (and add driver "supports" flag so index drivers can advertise whether or not they support the API.)

• CLI entry points for creating, updating, dropping and listing spatial indexes.

• Tests added / passed

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

[codecov]

Codecov Report

Patch coverage: 89.60% and project coverage change: +0.05% 🎉

Comparison is base (9ef6456) 85.18% compared to head (2f091fd) 85.23%.

Additional details and impacted files

@@               Coverage Diff               @@
##           develop-1.9    #1481      +/-   ##
===============================================
+ Coverage        85.18%   85.23%   +0.05%     
===============================================
  Files              130      131       +1     
  Lines            14358    14532     +174     
===============================================
+ Hits             12231    12387     +156     
- Misses            2127     2145      +18     

Files Changed	Coverage Δ
datacube/index/postgres/index.py	100.00% <ø> (+1.05%)	⬆️
datacube/index/abstract.py	89.26% <71.42%> (+0.09%)	⬆️
datacube/scripts/spindex.py	88.96% <88.96%> (ø)
datacube/drivers/postgis/_connections.py	83.57% <93.33%> (+0.11%)	⬆️
datacube/drivers/postgis/_api.py	87.34% <100.00%> (ø)
datacube/drivers/postgis/_spatial.py	94.02% <100.00%> (-0.09%)	⬇️
datacube/index/postgis/_datasets.py	75.12% <100.00%> (ø)
datacube/index/postgis/index.py	100.00% <100.00%> (ø)
datacube/scripts/cli_app.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[Ariana-B]

Lgtm :)