fix: change camelcase to snakecase in all private functions

[cadenmyers13]

Since these are private functions, no deprecation warning is needed. This was my process:

1. scroll through a file until i found a private function that was camel case (ex. _myPrivateFunction())
2. do a global search of that function
3. do a global replacement of that function (ex. _myPrivateFunction() to _my_private_function())
4. add and commit the change with the commit message being the name of the new function (ex. _my_private_function)
5. repeat

I did this for all files under src including the pdf and sas (will have to update in diffpy.cmipdf too but should be easy). After a local installation, all tests passed (see below). To be extra sure, I ran all cmi script examples too with the local installation and they all worked.

[codecov]

Codecov Report

❌ Patch coverage is 96.36364% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.60%. Comparing base (256d9e1) to head (32d69ab).
⚠️ Report is 60 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/test_objcrystparset.py	0.00%	1 Missing ⚠️
tests/test_sgconstraints.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #138   +/-   ##
=======================================
  Coverage   66.60%   66.60%           
=======================================
  Files          25       25           
  Lines        3171     3171           
=======================================
  Hits         2112     2112           
  Misses       1059     1059           

Files with missing lines	Coverage Δ
tests/conftest.py	86.66% <100.00%> (ø)
tests/test_contribution.py	99.51% <100.00%> (ø)
tests/test_pdf.py	43.29% <ø> (ø)
tests/test_recipeorganizer.py	99.69% <100.00%> (ø)
tests/test_objcrystparset.py	10.27% <0.00%> (ø)
tests/test_sgconstraints.py	10.06% <75.00%> (ø)

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[cadenmyers13]

@sbillinge ready for review

[sbillinge]

@cadenmyers13 nicely done there!