Write SDMX-ML elements in XSD schema order

[khaeru]

Closes #210

PR checklist

• Checks all ✅
• Update documentation N/A, bugfix
• Update doc/whatsnew.rst

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.94%. Comparing base (4116706) to head (47ba7a9).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #211      +/-   ##
==========================================
- Coverage   98.90%   97.94%   -0.97%     
==========================================
  Files          97       98       +1     
  Lines        8073     8088      +15     
==========================================
- Hits         7985     7922      -63     
- Misses         88      166      +78     

Files with missing lines	Coverage Δ
sdmx/format/xml/__init__.py	100.00% <100.00%> (ø)
sdmx/tests/writer/test_writer_xml.py	100.00% <100.00%> (ø)
sdmx/writer/xml.py	98.06% <100.00%> (ø)

... and 19 files with indirect coverage changes

[khaeru]

@3nz01 changing that one line I pointed to appears to resolve the issue—at least in the one test added on this branch.

Please let me know if you will be able to test the fix in your actual workflow, or not. Once I hear from you I will merge and make a patch release.

[3nz01]

@khaeru We weren't able to bring the entire project into our environment to test. However we did replicate the change you made on line 303 of sdmx/sdmx/writer/xml.py and it worked perfectly. From our POV the solution works and you can release the patch.

You might want to leave a note on the SIS-CC's gitlab saying that this has been taken care of. I don't mind doing it, but I figure you deserve the credit :)

Many thanks from the entire StatCan SDMX team.