Implement classical generator model in phasor-domain dynamics familly

[pelesh]

Implement second-order (classical) generator model as a separate model in Phasor Dynamics family of components. The model should inherit from PhasorDynamics::Component class. To complete:

• Generator model implementation.
• Proper CMake build configuration.
• Documentation in a form of README file with model equations.
• Unit tests.
• Example of usage.

[pelesh]

CC @abirchfield