feat: add off_center to erf_square

[yitchen-tim]

Description of changes:
Add an off_center parameter to allow offsetting the smoothed square toward left or right in a erf_square waveform.

Testing done:
tox

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (f87be27) to head (647d206).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1023   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          135       135           
  Lines         9035      9036    +1     
  Branches      2030      2030           
=========================================
+ Hits          9035      9036    +1     

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