usage:  [P, w] = __power (b, a, [, nfft [, Fs]] [, range] [, units])

Plot the power spectrum of the given ARMA model.

b, a: filter coefficients (b=numerator, a=denominator)
nfft is number of points at which to sample the power spectrum
Fs is the sampling frequency of x
range is 'half' (default) or 'whole'
units is  'squared' or 'db' (default)
range and units may be specified any time after the filter, in either
order

Returns P, the magnitude vector, and w, the frequencies at which it
is sampled.  If there are no return values requested, then plot the power
spectrum and don't return anything.