Union support, including eu

[samlown]

• Support for l10n Union model.
• Adding European Union as example.\
• Added comments to Country Codes to make them easier to differentiate in code editor.

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added thorough tests with at least 90% code coverage.
• I've modified or created example GOBL documents the show my changes in use, if appropriate.
• When adding or modifying a tax regime or addon, I've added links to the source of the changes either structured or in the comments.
• I've run go generate . to ensure that Schemas and Regime data are up to date.
• All linter warnings have been reviewed and fixed.
• I've been obsessive with pointer nil checks to avoid panics.
• The CHANGELOG.md has been updated with an overview of my changes.
• Requested a review from @samlown.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.25%. Comparing base (74035a5) to head (5f267da).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #450      +/-   ##
==========================================
+ Coverage   83.22%   83.25%   +0.02%     
==========================================
  Files         243      245       +2     
  Lines       11913    11933      +20     
==========================================
+ Hits         9915     9935      +20     
  Misses       1638     1638              
  Partials      360      360              

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[cavalle]

LGTM!