dedicated minimal tests -- nodes.weld_edges()

[jGaboardi]

This PR:

• adds dedicated minimal tests for nodes.weld_edges()
• xref start testing + refactor [stub issue] #21, start filling up testing suite & docstrings #20
• adds testing
• adds docstrings and type hints

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.1%. Comparing base (592aef2) to head (9c4e7f5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #58   +/-   ##
=====================================
  Coverage   98.1%   98.1%           
=====================================
  Files          6       6           
  Lines        894     894           
=====================================
  Hits         877     877           
  Misses        17      17           

Files with missing lines	Coverage Δ
sgeop/nodes.py	97.5% <100.0%> (ø)