dedicated minimal tests -- nodes.split()

[jGaboardi]

This PR:

• adds dedicated minimal tests for nodes.split()
• adds a nodes._snap_n_split() helper
• adds docstrings and type hints for those
• adds testing for those
• xref start testing + refactor [stub issue] #21, testing, refactor, & docstrings #20

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.1%. Comparing base (8d82df6) to head (b44348d).
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #55   +/-   ##
=====================================
  Coverage   98.1%   98.1%           
=====================================
  Files          6       6           
  Lines        893     894    +1     
=====================================
+ Hits         876     877    +1     
  Misses        17      17           

Files with missing lines	Coverage Δ
sgeop/nodes.py	97.5% <100.0%> (+<0.1%)	⬆️

... and 1 file with indirect coverage changes