Geospatial search api and ready for 1.9.0-rc9 release

[SpacemanPaul]

Reason for this pull request

Spatial search conventions differed between the core API (e.g. dc.load) and index API (e.g. dc.index.datasets.search) and were implemented similarly but differently in both places.

Proposed changes

• Keep dc.load() API unchanged
• Update index spatial search API to match core API (Use geopolygon instead of geometry)
• Consolidate spatial search argument handling out of the core API and into the lower shared levels of the index layer.
• Ugly hack to get netcdf writing with all recent versions of odc-geo

• Update whats_new.rst ready for 1.9.0-r 9 release.

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 99.23664% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.28%. Comparing base (13c06d6) to head (9bc864d).

Files	Patch %	Lines
datacube/drivers/netcdf/writer.py	88.88%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1611 +/- ## =============================================== + Coverage 85.16% 85.28% +0.11% =============================================== Files 141 142 +1 Lines 15574 15585 +11 =============================================== + Hits 13264 13291 +27 + Misses 2310 2294 -16 ```

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