Allow datetime timestamps for datafiles; use GCS custom time field for Datafile.timestamp

[cortadocodes]

Contents

New Features

• Allow datetime and posix timestamps for Datafile.timestamp
• Add Datafile.posix_timestamp property

Minor improvements

• Close Use GCS custom_time as timestamp metadata storage #149: use Google Cloud Storage custom_time metadata field for storing Datafile.timestamp
• Get datetime objects directly from blob instead of parsing string serialisations
• Add time utils module
• Add hash preparation function to Hashable for datetime instances

Quality Checklist

• New features are fully tested (No matter how much Coverage Karma you have)

[codecov-commenter]

Codecov Report

Merging #154 (8b5822f) into release/0.1.17 (89b0fa9) will decrease coverage by 0.15%.
The diff coverage is 91.66%.

@@                Coverage Diff                 @@
##           release/0.1.17     #154      +/-   ##
==================================================
- Coverage           94.61%   94.45%   -0.16%     
==================================================
  Files                  54       55       +1     
  Lines                1542     1570      +28     
==================================================
+ Hits                 1459     1483      +24     
- Misses                 83       87       +4     

Impacted Files	Coverage Δ
octue/resources/datafile.py	96.02% <85.00%> (-2.51%)	⬇️
octue/cloud/storage/client.py	96.29% <100.00%> (+0.35%)	⬆️
octue/mixins/hashable.py	100.00% <100.00%> (ø)
octue/utils/time.py	100.00% <100.00%> (ø)

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[thclark]

Looks good overall, but there's a merge conflict since merging the GCS metadata fix branch... and test coverage of the Datafile resource has dropped a bit.

Can you improve the coverage and sort the merge conflict please?

[thclark]

Also, @cortadocodes just to check, is it possible for the timestamp to be None? That's a legitimate possibility in doing from_cloud with a cloud object whose metadata is not yet in place.