OpPoDyn: stateless powerflow

[hexaeder]

using #255 , we should be able to define a "stateless" powerflow solution

pf_object = powerflow(nw, s0=get_defaults(nw_static))
# pf object might be a dataframe or such for manual inspection

pf_interface_values = get_interface_states(pf_object)
# this is essentially a dict of dict of interface variables, i.e.
# VIndex(1) => Dict(:u_r=1.0, :u_i=0.0, :i_r=0.1, :i_i=0.1)
# VIndex(2) ...
# for all edges and vertices

s0 = initialize(nw, pf_interface_values)

initialize (no !) could essentially do something like

s0 = NWState(nw, fillp=NaN, fillu=NaN)
for c in component_idxs
    model = nw[c]
    metadata_defaults = get_defaults(model)
    defaults = merge(metadata_defaults, pf_interface_values[c])
    guesses = get_guesses(model)
    inits, res = initialize_component(nw[c], defaults, guesses)
    @assert res is small 
    initialized_state = merge(defaults, inits)
    s0[c] = initialized_state # fill flat state representation with initialized state
end

The additional constraints/explicit initialization is considered inside initialize_component according to #255.