Finalize IEEE 39-bus example

[pelesh]

Summary

The IEEE 39-bus example reads input data and allocates the model but does not run simulation. Need to finalize this example.

Rationale

This is a commonly used model in research and for testing.

Description

Follow the implementation of ThreeBusBasicJson.cpp as the template.

Additional information

No response

[lukelowry]

I have faithfully implemented the complete IEEE 39-Bus case in my branch lukel/cases-add-dev. Given the small model and the growing number of cases, we need to add the IEEEST stabilizer to PR #365 to mark this case as complete. Otherwise, once that is merged, this will get unblocked.

The only thing GridKit does not do correctly is model phase shifters or LTC (transformers) - and since our Branch class is not general enough to allow for variations of a branch with additional functionality such as tap ratios or phase shifts, the IEEE case must be modified to have a tap of 1. This is a power-flow concern only for now. I think its a good time to improve the expressability of the Branch class, by generalizing to BranchBase, much like the BusBase.