Refactor phasor dynamics bus and system classes #604

Summary

There are a few features that need to be implemented in order to enable PhasorDynamics::SystemModel partitioning and subsequently implementation of an interface to partitioned solvers. See #143 and #323, as well as #463.

@superwhiskers @alexander-novo @nkoukpaizan @abdourahmanbarry @lukelowry @Steven-Roberts

Rationale

Bus (or rather BusBase class) and SystemSolver class need to provide features to support partitioning a system into subsystems. Enabling hierarchical modeling would allow applying different methods from a multimethod solver to different system partitions.

Description

Some thoughts how to proceed:

• Consider adding feature to the SystemModel to take other SystemModel instances as components.
• Consider enabling SystemModel to identify buses that are used for external connections so they can be used as components in a larger system.
• Add subsystem specification to JSON input file format.
• Remove pointers to buses from component model constructors. Set connection to buses in a similar manner as connections to signal nodes.
• Consider adding signal-connected buses as proposed in Add BusSignalVoltageOut and BusSignalVoltageIn bus models #602.

Additional information

This issue is to discuss prep work needed for partitioned models. PR #573 illustrates well what support is needed for hierarchical modeling.