Add SDMX input/output

[khaeru]

Closes #21.

PR checklist

• Complete implementation
  • Handle both SDMX 2.1 and 3.0 classes.
  • Detect or accept observation_dimension.
  • Add write_report() override.
• Checks all ✅
• Update documentation
• Update doc/whatsnew.rst

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ac3ba5e) 100.00% compared to head (691a80f) 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##              main      #115    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           48        50     +2     
  Lines         4118      4227   +109     
==========================================
+ Hits          4118      4227   +109     

Files	Coverage Δ
genno/compat/sdmx/__init__.py	100.00% <100.00%> (ø)
genno/compat/sdmx/operator.py	100.00% <100.00%> (ø)
genno/compat/sdmx/util.py	100.00% <100.00%> (ø)
genno/core/attrseries.py	100.00% <100.00%> (ø)
genno/core/computer.py	100.00% <100.00%> (ø)
genno/tests/compat/test_sdmx.py	100.00% <100.00%> (ø)