Add Additive/Vector/Threshold util APIs and tests

[Kyle-Ye]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.11765% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.55%. Comparing base (368d33c) to head (85d63c8).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
Sources/OpenGestures/Util/VectorArithmetic.swift	77.77%	4 Missing ⚠️
Sources/OpenGestures/Util/LocationContaining.swift	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   77.62%   78.55%   +0.92%     
==========================================
  Files          65       71       +6     
  Lines        1712     1814     +102     
==========================================
+ Hits         1329     1425      +96     
- Misses        383      389       +6     

Flag	Coverage Δ
ios	72.28% <86.36%> (+0.45%)	⬆️
macos	78.55% <94.11%> (+0.92%)	⬆️

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