Implement computed / observed states

[MasonProtter]

From the tests:

[...]
function GraphDynamics.subsystem_differential(sys::Subsystem{Oscillator}, F, t)
    (;x, v, x₀, m, k) = sys
    dx = v
    dv = (F - k*(x - x₀))/m
    SubsystemStates{Oscillator}((;x=dx, v=dv))
end
GraphDynamics.computed_properies(::Subsystem{Oscillator}) = (;ω₀ = ((;m, k),) -> √(k/m))

[...]

@testset "solutions" begin
    sol = solve_particle_osc(;x1=1.0, x2=-1.0)

    k = GraphDynamics.getp(sol, :osc₊k)(sol)
    m = GraphDynamics.getp(sol, :osc₊m)(sol)
    @test sol[:osc₊ω₀, end] == √(k/m)
end

I.e. this lets us access "computed quantities" at the solution level. Don't currently support full observed expressions, only pre-defined symbols for now.