Unify interface with SDDP.jl

[odow]

Ref https://github.com/odow/SDDP.jl

[odow]

• Get rid of constraints and terminal cost.

  m = SPModel(stages=3, sense=:Min) do sp, t
  @states(sp, begin
  end)
  @controls(sp, begin
  end)
  @noises(sp, begin
  end)
  dynamics!(sp) do out_state, in_state, control, noise
      if control[u] == 1
          return DynamicProgramming.INFEASIBLE
      end
      out_state[x] = in_state[x]+1
      return noise[w]
  end
  end