Refactor relationships

[bkryza]

Fixes #310

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.

Project coverage is 90.54%. Comparing base (43482c9) to head (9c8beac).
Report is 1 commits behind head on master.

Files	Patch %	Lines
src/class_diagram/model/diagram.cc	80.00%	1 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #311   +/-   ##
=======================================
  Coverage   90.54%   90.54%           
=======================================
  Files         123      121    -2     
  Lines       13803    13722   -81     
=======================================
- Hits        12498    12425   -73     
+ Misses       1305     1297    -8     

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