MNT: Refactor extraction code

[jsheunis]

This PR:

• moves the get_required_content method to the base class MetadataExtractorBase so that both FileMetadataExtractor and DatasetMetadataExtractor can make use of it (addresses Should get_required_content() also be available for file-level extractors? #287)
• refactors extract.py in order to deduplicate code, and updates related tests (addresses MNT: refactor extract.py to deduplicate code #366):
  • merges do_file_extraction() and do_dataset_extraction() into a single function do_extraction(), with a new argument extractor_type, which would be either 'file' or 'dataset'.
  • merges perform_file_metadata_extraction() and perform_dataset_metadata_extraction() into a single function perform_metadata_extraction()

[christian-monch]

Thx @jsheunis .

Only one thing. Let's move the extractor_type variable into the ExtractionArguments-class (see comments in the code)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c741d49) 86.28% compared to head (9eb3a51) 86.28%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #367   +/-   ##
=======================================
  Coverage   86.28%   86.28%           
=======================================
  Files          88       88           
  Lines        4842     4828   -14     
=======================================
- Hits         4178     4166   -12     
+ Misses        664      662    -2     

Impacted Files	Coverage Δ
datalad_metalad/extract.py	88.21% <100.00%> (-0.07%)	⬇️
datalad_metalad/extractors/base.py	88.00% <100.00%> (+1.33%)	⬆️
datalad_metalad/tests/test_extract.py	99.64% <100.00%> (-0.01%)	⬇️

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[jsheunis]

Thanks, good suggestion. I committed some of your suggestions, added more changes in 9eb3a51, including updating tests to reflect the changed parameter.

[jsheunis]

@christian-monch I'll leave the merging to you once you're happy with the state of the PR.

[christian-monch]

Thx a lot @jsheunis