Create README file documenting classical generator model (#122)

* Creat README file for classical generator model.

Author: pelesh

src/Model/PhasorDynamics/SynchronousMachine/GenClassical/README.md

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+# Classical Generator
+
+An electrical machine model with two differential variables (i.e. second order
+model) is often called classical generator model. While its predicitve ability
+is limited, it is useful for studies of grid network properties. Mathematically,
+it is equivalent to a driven damped pendulum model.
+
+## Model Parameters
+
+Symbol      | Units   | Description                     | Note
+------------|---------|---------------------------------|----------------------
+$\omega_0$  | [rad/s] | synchronous frequency           |
+$H$         | [s]     | rotor inertia                   |
+$D$         | [p.u.]  | damping coefficient             |
+$R_a$       | [p.u.]  | winding resistance              |
+$X_{dp}$    | [p.u.]  | machine reactance parameter     |
+
+### Model Derived Parameters
+
+- $g = \dfrac{R_a}{R_a^2 + X_{dp}^2}$
+- $b = \dfrac{-X_{dp}}{R_a^2 + X_{dp}^2}$
+
+<br>
+
+## Model Variables
+
+### Internal Variables
+
+#### Differential
+
+Symbol      | Units   | Description         | Note
+------------|---------|---------------------|----------------------
+$\delta$    | [rad]   | machine power angle |
+$\omega$    | [p.u]   | machine speed       | Optionally read by a governor or a stabilizer component
+
+#### Algebraic
+
+Symbol  | Units  | Description                         | Note
+--------|--------|-------------------------------------|-------------
+$T_{e}$ | [p.u.] | electrical torque                   |
+$I_r$   | [p.u.] | machine real injection current      | read by bus
+$I_i$   | [p.u.] | machine imaginary injection current | read by bus
+
+Note: All three can be expressed as function called by model equations. We add
+these as variables as they are needed for outputs.
+
+<br>
+
+### External Variables
+
+External variables enter component model equations but are owned by other
+components. The other components also provide equations needed to have a
+balanced system of equations. 
+
+#### Differential
+
+None.
+
+#### Algebraic
+
+Symbol | Units   | Description                   | Note
+-------|---------|-------------------------------|----------------------
+$V_r$  | [p.u.]  | machine bus real voltage      | owned by a bus object
+$V_i$  | [p.u.]  | machine bus imaginary voltage | owned by a bus object
+$P_m$  | [p.u.]  | mechanical power input        | owned by governor, constant if no governor is connected to the machine
+$E_p$  | [p.u.]  | field winding voltage         | owned by exciter, constant if no exciter is connected to the machine
+
+<br>
+
+
+## Model Equations
+
+### Differential Equations
+
+```math 
+\begin{aligned}
+\dot{\delta} &= (\omega - 1) \cdot \omega_0 \\
+\dot{\omega} &= \frac{1}{2H}\left( \frac{P_{m} - D(\omega - 1)}{\omega}   - T_{e}\right)
+\end{aligned}
+```
+
+### Algebraic Equations
+
+```math
+\begin{aligned}
+    0 &= T_{e} - \frac{1}{\omega}\left( g E_p^2 - E_p \left[(gV_r - bV_i)\cos\delta + (bV_r + gV_i)\sin\delta \right]\right)\\
+    0 &= I_r + gV_r - bV_i - E_p(g \cos\delta - b \sin\delta) \\
+    0 &= I_i + gV_r + bV_i - E_p(b \cos\delta + g \sin\delta)
+\end{aligned}
+```
+As noted earlier, all three algebraic equations can be expressed as functions
+and substituted directly in the component and bus equations, respectively. We
+use redundant variables for modeling convenience.
+
+<br>
+
+## Initialization
+
+To initialize the model, given bus voltages $V_r$, $V_i$, and initial generator
+injection active and reactive power, $P$ and $Q$, we take following steps to
+initialize the system:
+
+First compute injection currents from initial power injection power and bus
+voltages:
+```math
+\begin{aligned}
+I_r &= \frac{PV_r + QV_i}{V_r^2 + V_i^2} \\
+I_i &= \frac{PV_i - QV_r}{V_r^2 + V_i^2}
+\end{aligned} 
+```
+
+Next compute field winding voltage and machine angle:
+```math
+\begin{aligned}
+E_r &= \frac{ g(I_r + gV_r - bV_i) + b (I_i + bV_r + gV_i)   }{g^2 + b^2} \\
+E_i &= \frac{ -b(I_r + gV_r - bV_i) + g (I_i + bV_r + gV_i)   }{g^2 + b^2} \\
+E_p &= \sqrt{E_r^2 + E_i^2} \\
+\delta &= \arctan \dfrac{E_i}{E_r}
+\end{aligned}
+```
+
+Set machine speed to the synchronous speed:
+```math
+\omega = 1
+```
+
+Now, we can compute electrical torque and set mechanical torque to be equal
+to the electrical.
+```math
+\begin{aligned}
+T_{elec} &= gE_p^2 - E_p \left[ (gV_r -  bV_i ) \cos\delta + (bV_r + gV_i )\sin\delta \right] \\
+P_{mech} &= T_{elec} 
+\end{aligned} 
+```
+
+With this, we initialize the machine at a steady state.