Refactor PhasorDynamics hierarchy to consolidate duplicate code

[PhilipFackler]

Description

• Refactor class hierarchy for buses and components to consolidate code for signals and monitors
• Encapsulate implementation of SystemModel
• Use new style for template parameters and make public interface types consistent (addresses Use different style for template parameters and public interface types #366 for phasor dynamics code)

Proposed changes

This inserts a class (ConnectedElement) into the phasor dynamics hierarchy which will manage signal node ports and monitors. This new class takes its child class as its template parameter and uses traits to get more detailed type information (for variable enums, etc...) and also to customize the inheritance graph. Each leaf node device type must inherit from ConnectedElementTraits with itself as the template argument and define a customization of ConnectedElementTraits with the appropriate type information. The InterfaceT member determines which interface is inherited. For example...

For Bus

    template <typename ScalarP, typename IdxP>
    struct ConnectedElementTraits<Bus<ScalarP, IdxP>>
    {
      using BusT = Bus<ScalarP, IdxP>;

      using ElementT           = BusT;
      using ScalarT            = ScalarP;
      using IdxT               = IdxP;
      using RealT              = typename ScalarTraits<ScalarT>::RealT;
      using ModelDataT         = BusData<RealT, IdxT>;
      using InternalVariablesT = BusInternalVariables;
      using ExternalVariablesT = BusExternalVariables;
      using InterfaceT         = BusBase<ScalarT, IdxT>;  // using BusBase interface
    };

    template <typename ScalarP, typename IdxP>
    class Bus : public ConnectedElement<Bus<ScalarP, IdxP>>
    { /*...*/ };

// The inheritance graph for Bus is then:
// Bus <- ConnectedElement<Bus> <- BusBase <- GridElement <- Model::Evaluator

And for Branch

    template <typename ScalarP, typename IdxP>
    struct ConnectedElementTraits<Branch<ScalarP, IdxP>>
    {
      using BranchT = Branch<ScalarP, IdxP>;

      using ElementT           = BranchT;
      using ScalarT            = ScalarP;
      using IdxT               = IdxP;
      using RealT              = typename ScalarTraits<ScalarT>::RealT;
      using ModelDataT         = BranchData<RealT, IdxT>;
      using InternalVariablesT = BranchInternalVariables;
      using ExternalVariablesT = BranchExternalVariables;
      using InterfaceT         = Component<ScalarT, IdxT>;
    };

    template <typename ScalarP, typename IdxP>
    class Branch : public ConnectedElement<Branch<ScalarP, IdxP>>
    { /* ... */ };

// The inheritance graph for Branch is then:
// Branch <- ConnectedElement<Branch> <- Component <- GridElement <- Model::Evaluator

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• [1] The new code follows GridKit™ style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• [2] 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.

1. This PR changes the style guidelines with respect to template parameters and type aliases.

2. This PR was developed on top of Refactoring phasor dynamics constituent models #381 and will need to be rebased.

Further comments

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