a prior distribution for it. A convenient
% choice is the conjugate inverse chi-squared distribution, which
% allows Gibbs sampling step for sigma2 after every
% Metropolis-Hastings update for the other parameters. This is
% acchieved by |options.updatesigma=1|, below. The default prior is
% uninformative, but we can set the prior parameters with the
% following options. Option |model.N| for the number of observatios is
% needed for 'updatesigma', if it is not given, the code tries to
% guess |N| from the d