Use "!" for fcns that modify inputs about powermodels.jl

[mlubin]

As a counterpoint we've avoided these in JuMP because they look confusing to non-Julia programmers and we want JuMP code to be something pretty that you can put on a slide without needing to explain too much. MathProgBase has them because it's meant for people who know what they're doing. So there's definitely a decision to be made on whether they're wanted here or not.

[ccoffrin]

Both are good points. In this case, nearly every function would have ! appended to. Let me think on it.

[ccoffrin]

After thinking on this for a bit, my current preference is to not use ! for now. My rational is,

1. I do agree with @mlubin's point, many users of this code may be non-julia experts. Having to put ! on everything will seem confusing / tedious to them.
2. One of the design goals is to make PowerModels feel like JuMP. So the abstractions will seem familiar to users who know JuMP. Hence, we should follow the JuMP convention and not to use !.

In the future, if we end up having a "core" abstraction layer, like MathProgBase, we will use the Julia ! convention.

[kersulis]

Makes sense, I think this is the right call.