Return h-Tilbert transform of a periodic sequence x.
If x_j and y_j are Fourier coefficients of periodic functions x and y, respectively, then
y_j = sqrt(-1)*coth(j*h*2*pi/period) * x_j y_0 = 0
(theoretically oo-Tilbert == Hilbert). For even len(x), the Nyquist mode of x is taken zero.