Fix shape derivative of mollified edge-edge contact

[zfergus]

Description

Fix the shape derivative of mollified edge-edge contact.

Additionally, this makes the shape derivative computation object-oriented.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

New unit tests of mollifier and derivatives.

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

Merging #76 (739dc97) into main (8768a78) will increase coverage by 0.13%.
The diff coverage is 97.90%.

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
+ Coverage   95.64%   95.77%   +0.13%     
==========================================
  Files          88       88              
  Lines        7328     7439     +111     
==========================================
+ Hits         7009     7125     +116     
+ Misses        319      314       -5     

Files	Coverage Δ
src/ipc/collisions/collision_constraint.hpp	100.00% <ø> (ø)
src/ipc/collisions/collision_constraints.hpp	100.00% <ø> (ø)
src/ipc/collisions/edge_edge.cpp	98.79% <100.00%> (+0.49%)	⬆️
src/ipc/collisions/edge_edge.hpp	100.00% <ø> (ø)
src/ipc/distance/edge_edge_mollifier.cpp	100.00% <100.00%> (ø)
src/ipc/collisions/collision_constraints.cpp	84.23% <96.55%> (+0.29%)	⬆️
src/ipc/collisions/collision_constraint.cpp	92.98% <92.59%> (-0.36%)	⬇️

... and 2 files with indirect coverage changes