refactor: bring diffpy.srfit.util up to standards

[cadenmyers13]

This PR:

1. Deprecates nine camelCase util names in favour of snake_case (observer, tag, name, input and sort-key helpers) and switch all internal callers.
2. Restyles the tagmanager and weakrefcallable tests
3. Adds tests for the untested util modules
4. Puts the deprecation tests under tests/deprecated for easy removal later
5. Rewrites the util docstrings in NumPy style.

[codecov]

Codecov Report

❌ Patch coverage is 99.69136% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.39%. Comparing base (a9eae82) to head (91e2267).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
tests/deprecated/test_util_deprecations.py	98.78%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #220       +/-   ##
===========================================
+ Coverage   71.63%   83.39%   +11.76%     
===========================================
  Files          25       32        +7     
  Lines        3437     4343      +906     
===========================================
+ Hits         2462     3622     +1160     
+ Misses        975      721      -254     

Files with missing lines	Coverage Δ
tests/conftest.py	91.54% <100.00%> (+3.13%)	⬆️
tests/test_inpututils.py	100.00% <100.00%> (ø)
tests/test_nameutils.py	100.00% <100.00%> (ø)
tests/test_observable.py	100.00% <100.00%> (ø)
tests/test_tagmanager.py	100.00% <100.00%> (+1.36%)	⬆️
tests/test_util.py	100.00% <100.00%> (ø)
tests/test_visitors.py	99.19% <100.00%> (ø)
tests/test_weakrefcallable.py	100.00% <100.00%> (+1.08%)	⬆️
tests/deprecated/test_util_deprecations.py	98.78% <98.78%> (ø)

... and 12 files with indirect coverage changes

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[cadenmyers13]

@sbillinge ready for review. There are a lot of changes on this PR but most are docstring and deprecation work. The meaningful changes are the tests. Claude refactored some tests that were using unittest and wrote tests for:

• argbinders.py
• inpututils.py
• nameutils.py
• observable.py
• sortKeyForNumericString (in util/__init__.py)

[cadenmyers13]

The tests in this file are identical to the test for the 9 deprecated functions plus a test for catching the deprecation message