Add recordLinkage function and tests

[MuellerRoman]

Description

This pull request adds a new recordLinkage() function implementing Global Distance-Based Record Linkage (GDBRL) following Herranz et al. (2015).

What is included

• New recordLinkage() function for one-to-one global record linkage via the Hungarian algorithm
• Support for "gower", "euclidean", and "manhattan" distances
• Support for na_action = "ignore" and na_action = "fail"
• Optional return of the pairwise cross-dataset distance matrix

Unit tests covering:

• published example cases
• distance matrix calculations
• weighted and unweighted linkage
• Hungarian assignment behavior
• input validation
• missing-value handling
• ordered/unordered factor harmonization

Notes

• At present, the function supports data.frame inputs only; sdcObj objects are not yet supported!
• Files added:
  - R/recordLinkage.R
  - tests/testthat/test_recordLinkage.R
  - generated Rd documentation

Reference

Herranz, J., Nin, J., Rodríguez, P., and Tassa, T. (2015). Revisiting distance-based record linkage for privacy-preserving release of statistical datasets. Data & Knowledge Engineering, 100, 78–93. https://www.sciencedirect.com/science/article/pii/S0169023X15000543