Change const Eigen::Matrix& to Eigen::ConstRef<Matrix>

[zfergus]

Description

Change const Eigen::Matrix& to Eigen::ConstRef<Matrix> for better compatibility and fewer copies.

Drop scalar templating in:

• local_to_global
• CollisionStencil::dof
• CollisionStencil::vertices

Type of change

• Enhancement (non-breaking change which improves existing functionality)

How Has This Been Tested?

Existing unit tests.

Test Configuration:

• OS and Version: macOS 15.3.2
• Compiler and Version: Apple Clang 16.0.0

Checklist

• I have followed the project style guide
• My code follows the clang-format style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[codecov]

Codecov Report

Attention: Patch coverage is 91.37931% with 5 lines in your changes missing coverage. Please review.

Project coverage is 96.45%. Comparing base (9c56d11) to head (aaca2d4).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ipc/collisions/normal/normal_collision.cpp	25.00%	3 Missing ⚠️
src/ipc/broad_phase/spatial_hash.cpp	50.00%	1 Missing ⚠️
src/ipc/collisions/normal/normal_collision.hpp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #157      +/-   ##
==========================================
+ Coverage   96.36%   96.45%   +0.09%     
==========================================
  Files         102      102              
  Lines        7758     7760       +2     
==========================================
+ Hits         7476     7485       +9     
+ Misses        282      275       -7     

Flag	Coverage Δ
unittests	96.45% <91.37%> (+0.09%)	⬆️

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