Enzyme Jacobian for PhasorDynamics::Load within GridKit

[nkoukpaizan]

Description

Enzyme Jacobian for PhasorDynamics::Load within GridKit. This is done within the model library directly.

Proposed changes

• Added a wrapper in src/AutomaticDifferentiation/Enzyme/ to compute the Jacobians. This requires the residual wrapper to be in global namespace, but other definitions can be inside the GridKit::Enzyme namespace.
• Moved most of the implementation for the PhasorDynamics::Load into LoadImpl.hpp. This file in then included in Load.cpp (which calls Enzyme and provides a Jacobian) and LoadDependencyTracking.cpp, such that the two automatic differentiation approaches are compiled in different compilation units.
• Added a enzyme_jacobian unit test in LoadTest.hpp.
• Extended the use of templates in examples/Enzyme/ in preparation for converting these into unit test.
• Made the EnzymeVector tests more interesting to yield a lower triangular Jacobian rather than a diagonal one.
• Moved src/LinearAlgebra/SparsityPattern/ to src/AutomaticDifferentiation/DependencyTracking/
• Placed COO_Matrix into the GridKit::LinearAlgebra namespace.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.

Further comments

• This is an example implementation for the PhasorDynamics::Load. Extending to other models will be straight forward, so I'd like some feedback here before extending.
• I wouldn't worry about the compliance of the code in examples/Enzyme/ for now. I will move this to test/UnitTests/Enzyme/.