deprecate and refactor: deprecate camelCase functions in diffpy.srfit.equation and refactor test for pytest format

[cadenmyers13]

As the title says, this PR:

1. deprecates the camelCase function sin diffpy.srfit.equation
2. refactors the equation tests to be up to date with pytest standards
3. Adds/updates numpy style docstrings

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.18%. Comparing base (a9eae82) to head (91fba7c).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #221       +/-   ##
===========================================
+ Coverage   71.63%   84.18%   +12.55%     
===========================================
  Files          25       33        +8     
  Lines        3437     4527     +1090     
===========================================
+ Hits         2462     3811     +1349     
+ Misses        975      716      -259     

Files with missing lines	Coverage Δ
tests/conftest.py	92.89% <100.00%> (+4.48%)	⬆️
tests/deprecated/test_equation_deprecations.py	100.00% <100.00%> (ø)
tests/test_builder.py	100.00% <100.00%> (ø)
tests/test_constraint.py	98.30% <100.00%> (+1.33%)	⬆️
tests/test_contribution.py	99.58% <100.00%> (+0.06%)	⬆️
tests/test_equation.py	100.00% <100.00%> (ø)
tests/test_literals.py	100.00% <100.00%> (+0.82%)	⬆️
tests/test_recipeorganizer.py	99.53% <100.00%> (-0.16%)	⬇️
tests/test_restraint.py	96.87% <100.00%> (ø)
tests/test_visitors.py	100.00% <100.00%> (+0.80%)	⬆️

... and 12 files with indirect coverage changes

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[cadenmyers13]

@sbillinge ready for review