steady state presolver block

[ikarib]

A few suggestions on presolver block:

1. allow specifying steady state constraints in one block:

@steadystate model begin
    K = 47.63
    N = ((1-α)*K^α)^(1/(α+γ))
end

2. direct assignments should not run solve1d with bisection algorithm
3. break up blocks of steady state equations in large models using MATLAB's dmperm equivalent (see IRIS implementation)

[bbejanov]

Thanks for the suggestions!