Add pivot function to handle modeledyears files

[mcllerena]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.76%. Comparing base (2f5f407) to head (586acee).

Files with missing lines	Patch %	Lines
src/r2x_core/processors.py	60.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   97.94%   97.76%   -0.19%     
==========================================
  Files          19       19              
  Lines        1024     1029       +5     
==========================================
+ Hits         1003     1006       +3     
- Misses         21       23       +2     

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[Copilot]

Pull Request Overview

This pull request adds a pivot functionality to handle modeledyears files by introducing a new transformation step in the tabular data processing pipeline.

• Added pl_pivot_on function to perform DataFrame pivoting based on configuration
• Integrated the pivot function into the existing transformation pipeline
• The function unpivots all columns and selects only the specified pivot column

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[Copilot]

Calling collect_schema() forces schema collection which can be expensive for large lazy frames. Consider using df.schema.names() or df.columns if available in the Polars version being used.

Suggested change

	all_columns = df.collect_schema().names()
	all_columns = df.columns

[pesap]

@mcllerena , can you add notes using numpy style on why we do this? Also, i think we have on the docstring on the pipeline the process, can you update the docstring as well?