New flash output

[pw0908]

Right now, our flash algorithms spit out the composition, moles and reduced gibbs free energy:

(x,n,G) = tp_flash(model,p,T,z)

What I'd like to propose is for the output to some sort of struct:

abstract type PhaseState end
struct TwoPhaseState <: PhaseState
    composition::Vector{Float64}
    volume::Vector{Float64}
    pressure::Float64
    temperature::Float64
    initial_composition::Vector{Float64}
    phase_fraction::Vector{Float64}
    gibbs_free_energy::Float64
end

Where now:

state = tp_flash(model,p,T,z)

We could then define bulk property calculations such that:

function volume(model::EoSModel,state::PhaseState)
        v = state.volume
        ϕ = state.phase_fraction
        return sum(ϕ.*v)
end