Create README file documenting classical generator model

[pelesh]

Description

Created a README file specifying classical generator model parameters, internal and external variables, governing equations and initialization procedure.

Partially addresses #73

@abdourahmanbarry @reid-g @alexander-novo @nkoukpaizan

Proposed changes

• Defines parameters and "derived parameters" used in classical generator model equations.
• Defines internal and external model variables.
• Fixes mistakes in classical generator model in Gen classical #94.
• Specifies handling signal flow variables.
• Proposes adding bus injection currents as generator internal variables. This increases size of the generator model but adds a modeling convenience.

Checklist

N/A

• 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.

Further comments

[abirchfield]

Looks great.

[shakedregev]

How does a README file fix a test error in PR #94 ?

[alexander-novo]

How does a README file fix a test error in PR #94 ?

It's a documentation fix unrelated to the test error. The earlier documentation for that model was wrong.