Refine _time_to_search_dims logic

[Ariana-B]

Reason for this pull request

Passing time as an int/float to dc.load was causing the query to return an unexpected date range due to the value being interpreted as seconds since epoch. Int/float is not documented as an accepted type for time in the first place and could be ambiguous.

Proposed changes

• Raise error if time is not provided as some form of datetime or a string

• Tweak and consolidate logic in _time_to_search_dims and _to_datetime

• Remove added millisecond in time range from like as it appears unnecessary

• Closes #xxxx

• Tests added / passed

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

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📚 Documentation preview 📚: https://datacube-core--1561.org.readthedocs.build/en/1561/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.78%. Comparing base (b6cebf5) to head (4bf8cb8).

❗ Current head 4bf8cb8 differs from pull request most recent head 78948f5. Consider uploading reports for the commit 78948f5 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1561   +/-   ##
========================================
  Coverage    91.78%   91.78%           
========================================
  Files          132      132           
  Lines        14641    14621   -20     
========================================
- Hits         13438    13420   -18     
+ Misses        1203     1201    -2     

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[SpacemanPaul]

LGTM. Thanks.