added coupling matrix

[APernatii]

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• CONSIDER adding a unit and/or fullscale test if your PR resolves an issue.
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[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.48%. Comparing base (c99ef30) to head (552c242).
Report is 2 commits behind head on feature/coupling.

Files with missing lines	Patch %	Lines
src/FEM/Coupling/Arlequin.jl	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@                 Coverage Diff                  @@
##           feature/coupling     #211      +/-   ##
====================================================
- Coverage             83.53%   83.48%   -0.05%     
====================================================
  Files                    74       74              
  Lines                  6018     6019       +1     
====================================================
- Hits                   5027     5025       -2     
- Misses                  991      994       +3     

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