Move PowerElectronics models to a namespace uniformly

[nkoukpaizan]

Description

This moves PowerElectronics models to a namespace uniformly.

Proposed changes

• Relevant models that were straight in GridKit:: are now in GridKit::PowerElectronics.
• This simplifies the older Circuit* and PowerElectronicsModel names
• Updated tests and examples accordingly

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.
• [N/A] The new code is documented.
• The feature branch is rebased with respect to the target branch.
• [N/A] 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

I recommend ignoring whitespaces when reviewing.

[abdourahmanbarry]

This looks good.