Set proper regime when alternative code is given

[cavalle]

Describe your changes

Sets the proper tax regime code (e.g. EL) when the alternative code is given (e.g. GR)

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [x] I have added thorough tests with at least 90% code coverage.
• [x] I've modified or created example GOBL documents the show my changes in use, if appropriate.
• [x] 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.
• [x] I've run go generate . to ensure that Schemas and Regime data are up to date.
• [x] All linter warnings have been reviewed and fixed.
• [x] I've been obsessive with pointer nil checks to avoid panics.
• [x] Requested a review from @samlown.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.88%. Comparing base (15397b4) to head (41f1403). Report is 6 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #399 +/- ## ========================================== + Coverage 78.82% 78.88% +0.05% ========================================== Files 217 217 Lines 10721 10722 +1 ========================================== + Hits 8451 8458 +7 + Misses 1914 1909 -5 + Partials 356 355 -1 ```

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