Reduce duplicate code into hidden _auxiliary module

[c-randall]

Description

Some functions had repeated logic across modules (e.g., inferring states, computing SOC, detecting pulses, etc.). A new auxiliary module was added for developers to put these helper functions into one place instead of scattering and repeating them all over. Depending on how useful some of these functions are, they may also be considered to be user-facing at a later point.

This is a smaller PR to initialize this new module. An additional PR will include functions to infer steps, cycles, add a cumulative steps column, and to detect pulses (mentioned above, but not included here).

Type of change

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that improves speed/readability/etc.)
• Bug fix (non-breaking change which fixes an issue)
• Chore (any other clean up, maintenance, etc.)

Key checklist:

• No style issues: $ nox -s linter [-- format]
• Code is free of misspellings: $ nox -s codespell [-- write]
• All tests pass: $ nox -s tests

Further checks:

• The documentation builds: $ nox -s docs.
• Code is commented, particularly in hard-to-understand areas.
• Tests are added that prove fix is effective or that feature works.

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.26%. Comparing base (da4e7c5) to head (df74e92).

Files with missing lines	Patch %	Lines
src/ampworks/hppc/_extract_impedance.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   60.16%   60.26%   +0.10%     
==========================================
  Files          42       43       +1     
  Lines        1998     2001       +3     
==========================================
+ Hits         1202     1206       +4     
+ Misses        796      795       -1     

Flag	Coverage Δ
unittests	60.26% <92.85%> (+0.10%)	⬆️

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