Improved gas flow formulations

[nhniina]

Current gas flow implementation using first order Taylor approximation and big M could be improved.

(Related to #1001)

[awgutierrez]

Using existing tools is the main idea.
A simple example containing the key points is shown here https://github.com/awgutierrez/LinearOnOffConstraints

[DillonJ]

@nhniina if I remember, wasn't there a model and some slides and/or code that was made available?

[nhniina]

Yes, I believe the example @awgutierrez shared above contains a simple model/code.