Improve efficiency of freqresp for state space systems

[olof3]

There is old code the improve the efficiency of freqresp for state space systems, but it is not used.

# Implements algorithm found in:
# Laub, A.J., "Efficient Multivariable Frequency Response Computations",
# IEEE Transactions on Automatic Control, AC-26 (1981), pp. 407-408.
function _preprocess_for_freqresp(sys::StateSpace)
    A, B, C, D = sys.A, sys.B, sys.C, sys.D
    F = hessfact(A)
    H = F[:H]::Matrix{Float64}
    T = full(F[:Q])
    P = C*T
    Q = T\B
    StateSpace(H, Q, P, D, sys.Ts, sys.statenames, sys.inputnames,
        sys.outputnames)
end

Would be good to include the code. Would be good to improve the test coverage of freqresp at the same time.