CellmlToolkit.jl: How to get observables?

[ChrisRackauckas]

@andreypikunov

prob = ODEProblem(ml, (0, 42.0))
sol = solve(prob)
sol[membrane₊i_Na]

prob = ODEProblem(ml, (0, 42.0))
sol = solve(prob)
sys = ml.sys
sol[@nonamespace sys.membrane₊i_Na]

This is missing docs so I just pinged @YingboMa to make sure the namespacing gets documented.