Add possibility to model shunt elements in the power grid simulator

[robinhenry]

This issue will track the addition of shunt elements to the power grid simulator, just like MATPOWER and other simulation packages do.

Background

Shunt elements were not originally included in gym-anm because we didn't want to over-complicate things for beginners with little experience in power system modeling. However, it seems that the feature would be useful to a number of people.

Feel free to react to this comment if you would like to see this feature added, too!

Plan

The goal is to add the possibility to model shunt elements in the power grid simulator. It will follow the same mathematical representation as used by MATPOWER and others: shunt elements (e.g., capacitors or inductors) will be modeled as a fixed impedance connected to ground at a specific bus.

More precisely, the modifications should follow equations (3.7) and (3.13) of the MATPOWER official documentation.

[robinhenry]

I'm closing this as there doesn't seem to be much interest in this new feature. We can open it back up if needed later.