Payment receipts and advances in Portugal

[cavalle]

• Adds support for payment advances and receipts in PT
• Changes the default unit to one in PT
• Fixes a panic when calculating a bill.Receipt without lines
• Changes bill.Receipt to allow empty codes while the invoice isn't signed (like in bill.Invoice)
• Calculates line indexes automatically in bill.Receipt

Pre-Review Checklist

• 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 to 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 the 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 85.92%. Comparing base (b86e5d8) to head (d32c8bb).
Report is 12 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   85.67%   85.92%   +0.24%     
==========================================
  Files         255      257       +2     
  Lines       12582    12803     +221     
==========================================
+ Hits        10780    11001     +221     
  Misses       1454     1454              
  Partials      348      348              

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[samlown]

Code looks great! I do think we should start moving all the documentation to the structure approach though. My objective is to remove README.md files completely, as everything should be defined inside the Go structures.

[samlown]

I think we can be a bit more efficient perhaps with the tables, but otherwise, this looks great! Juan will be happy too :-)