dedicated minimal tests for continuity.py

[jGaboardi]

This PR:

• provides dedicated minimal tests for continuity.py
• beefs up docstrings & type types
• small refactoring for DRYness
• resolves need to return coins in continuity.continuity()? #49

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.1%. Comparing base (89f6f8b) to head (49ad0d0).
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main     #50   +/-   ##
=====================================
  Coverage   98.1%   98.1%           
=====================================
  Files          6       6           
  Lines        891     893    +2     
=====================================
+ Hits         874     876    +2     
  Misses        17      17           

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