Separate internal and external residuals in PowerElectronics models

[alexander-novo]

Description

Partially addresses #353. Separates the internal and external residual evaluations of all PowerElectronics models so that the internal residuals of all components can be easily evaluated in parallel.

Proposed changes

Two new pure virtual functions have been added to CircuitComponent: evaluateInternalResidual() and evaluateExternalResidual(). There is now a final implementation for CircuitComponent::evaluateResidual() which just calls these two new functions.

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.
• 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

[pelesh]

Very well focused PR addressing the issue at hand. Please address minor comments before we merge it.

[pelesh]

I suggest merging target branch to develop first and then rebasing this to develop and merging it there.

[alexander-novo]

@pelesh Done