Create a 3-bus example for phasor dynamic simulations

[pelesh]

Using the same grid model as for power flow 3-bus example, create and document a phasor dynamics 3-bus example. Notable differences with the power flow model should include:

• Replacing power flow generators with GENROU component models.
• Replacing dynamic phaseor Bus and branch component models.
• Parameters and initial values need to be converted from polar to Cartesian values.

As a first stab, it is sufficient if:

• The example is implemented as hard-wired set of equations. No need for system composer at this time.
• The example uses finite difference Jacobian

The example equations need to be documented in a README file, similar to the power flow example.

CC @abirchfield