Routine mar1psd: To compute the power spectum by AR-model parameters.
Input parameters:
Ip : AR model order (integer)
ep : White noise variance of model input (real)
ts : Sample interval in seconds (real)
a : Complex array of AR parameters a(0) to a(Ip)
Output parameters:
psdr : Real array of power spectral density values
psdi : Real work array
in chapter 12
