Remove GridElement class

[pelesh]

Description

Simplify inheritance structure for PhasorDynamics buses and components.

Proposed changes

Remove intermediary GridElement class and let BusBase inherit directly from Evaluator. While this does introduce some minor code duplication, we expect bus and component classes to get more specialized and the common code between them to change going forward. As we move to implement EMT models, having simpler inheritance structure will help, as well.

Also, I expect BusBase class to be removed altogether and have Bus inherit directly from Evaluator. Class BusInfinite should be a standalone class, similar to SignalNode and not inherit from Evaluator at all.

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

[PhilipFackler]

It works :)