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!