spectral density for frequency using polynomial roots
builds two arrays (number of roots, number of frequencies)
arroots : ndarray
roots of ar (denominator) lag-polynomial
maroots : ndarray
roots of ma (numerator) lag-polynomial
w : array_like
frequencies for which spd is calculated
Notes
this should go into a function
statsmodels.sandbox.tsa.fftarma.ArmaFft.spdroots
statsmodels.sandbox.tsa.fftarma.ArmaFft.spdshift
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