Adding general forms of control

[luap-pik]

I want to document this approach to implement a general (e.g. non-local) form of control by defining a custom PowerGrid type.

The point is that the vertex (and edge) functions in NetworkDynamics cannot access the state vector for other components. Hence, the controller here is a wrapper around the rhs that has access to
the full state vector u. The result is then passed via the parameter field.

This functionality can be easily added, although it will probably deprecate when NetworkDynamics offers multilayer support (?).

In Any case, I think it's a good idea to document the process of defining custom grids in the official docs.

struct ControlledPowerGrid <: AbstractPowerGrid
    graph:: G where G <: AbstractGraph
    nodes
    lines
    controller # oop function
end

# Constructor
function ControlledPowerGrid(control, pg::PowerGrid, args...)
    ControlledPowerGrid(pg.graph, pg.nodes, pg.lines, (u, p, t) -> control(u, p, t, args...))
end

#rhs
function rhs(cpg::ControlledPowerGrid)
    open_loop_dyn = PowerGrid(cpg.graph, cpg.nodes, cpg.lines) |> rhs
    function cpg_rhs!(du, u, p, t)
        # Get controller state
        control_output = cpg.controller(u, p, t)
        # Calculate the derivatives, passing the controller state through
        open_loop_dyn(du, u, control_output, t)
    end
    ODEFunction{true}(cpg_rhs!, mass_matrix=open_loop_dyn.mass_matrix, syms=open_loop_dyn.syms)
end

For many methods, e.g. systemsize, the type doesn't matter as long as we keep the same fields as the PowerGrid type. Hence, it might make sense to dispatch on an AbstractPowerGrid to allow more flexibility without manually extending lot's of methods.