Add hierarchical PhasorDynamics system model with a unit test

[nkoukpaizan]

Description

This is an initial implementation for a PhasorDynamics hierarchical system model.
Relates to #604
Relates to #619
The approach taken here is a nested JSON format, and the PR focuses on the system model implementation. For (more) optimal partitioning (e.g., for solvers), this requires a pre-processing step to create this structure (future work).

Proposed changes

• The key methods enabling the recursion are SystemModel(const SystemModelData<RealT, IdxT>& data, const std::vector<BusT*>& external_buses) and void addSubsystem(SystemModel* subsystem).
• Only buses are currently supported for interactions between subsystems. Signals will be addressed later.
• Several methods in the system model (e.g., evaluateResidual()) gain a for (const auto& subsystem : subsystems_) to go down the hierarchy.
• A unit test exercises the proposed hierarchical system model.
• Fixed a couple of bugs (e.g., destructor calling and ordering of adding/connectiion components).

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.
• The CHANGELOG.md has been updated to reflect the changes. If this is a minor PR that is part of a larger fix already included in the file, state so.

Further comments

Draft to initiate a discussion. I have a functional TwoArea case using this approach.
I'll update the CHANGELOG and rebase as this gets closer to ready-to-merge.

cc @Steven-Roberts @alexander-novo @abdourahmanbarry