Add CircuitNode class to PowerElectronics

[KakeruUeda]

Description

This adds a CircuitNode class to the PowerElectronics module to separate nodes from circuit components, along with a unit test.

cc @nkoukpaizan

Proposed changes

• Added a CircuitNode class
• Added a unit test for the CircuitNode class

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here
to help! This is simply a reminder of what we are going to look for before
merging your code.

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

Further comments

This is a minimal change and I haven’t updated the system model or related examples yet. I’ll implement those changes later.

[pelesh]

Looks good!