Add .sum4all

[khaeru]

This PR adds some metadata/structural information from the SUM4All “Global Tracking Framework”. In particular, this includes the list of “indicators” from the appendix as a concept scheme, including the labels, names, descriptions, and preferred units of measurement.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.32%. Comparing base (651375e) to head (904b74f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
+ Coverage   93.93%   94.32%   +0.39%     
==========================================
  Files          64       67       +3     
  Lines        2768     2959     +191     
==========================================
+ Hits         2600     2791     +191     
  Misses        168      168              

Files with missing lines	Coverage Δ
transport_data/__init__.py	100.00% <ø> (ø)
transport_data/sum4all/__init__.py	100.00% <100.00%> (ø)
transport_data/sum4all/structure.py	100.00% <100.00%> (ø)
transport_data/tests/test_org.py	100.00% <100.00%> (ø)
transport_data/tests/test_sum4all.py	100.00% <100.00%> (ø)
transport_data/tests/util/test_pluggy.py	100.00% <100.00%> (ø)

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