Plot a non-state variable via Decapodes

[KevinDCarlson]

It's currently not possible to plot, say, v in the Navier-Stokes example rather than dv because only state variables get included in the solution provided by ODEs.jl. To resolve this one needs to search for or provide the operator computing a derived quantity to somebody in AlgebraicJuliaService. There's currently a commented test for two plot variables that will assess success on this issue.