EMT Universal Line Model Documentation

[lukelowry]

Description

Improves documentation for EMT line models and adds documentation for other models needed to implement the most capable version of the Universal Line Model (ULM).

Proposed changes

• Rename to Line documentation for LineLumped and LineDistributed, replacing the old Branch line docs (existing model documentation functionally unchanged)
• Add an Operators group for the reusable blocks the models build on
  • Rational (VectorFit, StateSpace) for empirical f.d. transmission line models
  • Shift (Delay, Propagation) for distributed parameter modeling
  • Add skeleton docs for Coordinate (Park, DQZ)
• Reorganize into Component (Bus, Line, Load, Source) and Operators
• Add diagrams for the line and operator models, and update the EMT README indexes and figure paths.

Checklist

• N/A All tests pass.
• N/A Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• N/A The new code follows GridKit™ style guidelines.
• N/A 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.

Further comments

A convention is needed to document components composed of other components. Especially how to document 'internal ports', which depends on how we go about implementation.

If the 'Operators' concept is disliked, we can move under Components. It's just a conceptual split.

[lukelowry]

@pelesh, I made this PR draft so you can see the range of complexity for the models and their needs.

An open-source ULM (LineDistributed) would be invaluable, especially if we scale well to large cases. What most authors call 'EMT' is LineLumped, which is just fancy phasor dynamics. ULM is where the prizes are! To my knowledge, no one has ever tried to model the ULM with adaptive stepping like this.

Discuss later.