Api cleanup

[SpacemanPaul]

Reason for this pull request

datacube/api/core.py is where the core user-facing API classes (e.g. Datacube) live.

Previously it contained minimal typehints and out of date docstrings.

Proposed changes

• Add typehints to all methods.

• Update docstrings.

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

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.62500% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 85.59%. Comparing base (7af7e49) to head (b44e066).

Files	Patch %	Lines
datacube/api/core.py	90.32%	6 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop-1.9 #1567 +/- ## =============================================== - Coverage 85.59% 85.59% -0.01% =============================================== Files 140 140 Lines 15366 15379 +13 =============================================== + Hits 13152 13163 +11 - Misses 2214 2216 +2 ```

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[SpacemanPaul]

Thanks Robbi!