cc_diff(x, a, b, period=2*pi) -> y
Return (a,b)-cosh/cosh pseudo-derivative of a periodic sequence x.
If x_j and y_j are Fourier coefficients of periodic functions x and y, respectively, then
y_j = cosh(j*a*2*pi/period)/cosh(j*b*2*pi/period) * x_j