Fix the order of params in a ODESystem

[YingboMa]

Unfortunately, we cannot assume the order of params in an ODE system. For instance, the Lotka-Volterra is

function lotka(du,u,p,t)
  x = u[1]
  y = u[2]
  du[1] = p[1]*x - p[2]*x*y
  du[2] = -p[3]*y + p[4]*x*y
end

and generated Jacobian function is

    function ##405(##406, u, p, t)
        #= /Users/yingboma/.julia/dev/ModelingToolkit/src/utils.jl:55 =#
        let (x₁, x₂, α₁, α₃, α₄, α₂) = (u[1], u[2], p[1], p[2], p[3], p[4])
            ##406[1] = (+)(α₁, (*)(x₂, α₂, -1))
            ##406[2] = (*)(x₂, α₄)
            ##406[3] = (*)(α₂, x₁, -1)
            ##406[4] = (+)((*)(α₃, -1), (*)(α₄, x₁))
        end
        #= /Users/yingboma/.julia/dev/ModelingToolkit/src/utils.jl:56 =#
        nothing
    end

.

Note that it generates

(x₁, x₂, α₁, α₃, α₄, α₂) = (u[1], u[2], p[1], p[2], p[3], p[4])

which is incorrect. The order which params appear is not necessarily the same with the order which the user supplies it. Hence, we have to have an optional argument for params that retains the order.

[YingboMa]

Ah, we could do it just by

de = modelingtoolkitize(prob)
jac = eval(ModelingToolkit.generate_jacobian(de...))

[codecov-io]

Codecov Report

Merging #160 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
+ Coverage   92.84%   92.85%   +0.01%     
==========================================
  Files          12       12              
  Lines         461      462       +1     
==========================================
+ Hits          428      429       +1     
  Misses         33       33

Impacted Files	Coverage Δ
src/systems/diffeqs/diffeqsystem.jl	90.16% <100%> (+0.08%)	⬆️

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