NF: NIDMResults metadata extractor draft

[mih]

Quick draft of a NIDM result metadata extractor. This is using a JSON-LD representation as input that is not very suitable for the search implementations we have.

• RF this to use an alternative representation outlined in Exploration protocol, desired metadata structure datalad#2753
• document queries that can be performed with the existing search implementation based on the extractor output -> Exploration protocol, desired metadata structure datalad#2753
• tests

[codecov-io]

Codecov Report

Merging #41 into master will decrease coverage by 0.64%.
The diff coverage is 79.71%.

@@            Coverage Diff             @@
##           master      #41      +/-   ##
==========================================
- Coverage   89.03%   88.38%   -0.65%     
==========================================
  Files          19       21       +2     
  Lines         930      999      +69     
==========================================
+ Hits          828      883      +55     
- Misses        102      116      +14

Impacted Files	Coverage Δ
..._neuroimaging/extractors/tests/test_nidmresults.py	100% <100%> (ø)
datalad_neuroimaging/extractors/nidmresults.py	65% <65%> (ø)

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[cmaumet]

Here is a minimal JSON file we can use for testing: https://raw.githubusercontent.com/cmaumet/nidmresults-spm/export_from_json/test/spmexport/ex_spm_full_example001/nidm_minimal.json

[mih]

Q @cmaumet: The metadata contains absolute paths, e.g.:

        "SearchSpaceMaskMap_atLocation": "/test/data/nidmresults-examples/spm_full_example001/mask.nii",

Do they have to be like this, or could they be relative to the "root" of the result structure? In the present form it is hard to locate the file in the ZIP once extracted to an arbitrary location.

[mih]

Replaced by psychoinformatics-de/datalad-hirni#102