write a function that returns the steady state values for the internal variables of a dynamic node instance

[timziebart]

similarly to #28 craete a function that does

getInternalSteadyStateVariables(n::SwingEq) = (ω = 0.,)

with generic functions that look something like

getInternalSteadyStateVariables(n::AbstractNodeDynamics)= n |> parametersof |> getInternalSteadyStateVariables

Note: Of course, instead of using NamedTuples, I strongly usggest to use a custom InternalSteadyState type (that also has the type as a parameter).