GARCH Constructor

Constructor for GARCH.

public GARCH(
   int p,
   int q,
   double[] y,
   double[] xguess
);

p: A int scalar containing the number of autoregressive (AR) parameters.
q: A int scalar containing the number of moving average (MA) parameters.
y: A double array containing the observed time series data.
xguess: A double array of length p + q + 1 containing the initial values for the parameter array.

Exception Type	Condition
ArgumentException	is thrown if the dimensions of `y`, and `xguess` are not consistent