Changing prior covariance notation for GaussianFixedCov

I found Gelman's notation for the GaussianFixedCov distribution rather confusing. He uses \Lambda_0 to denote a prior *covariance* matrix for \mu, but I typically associate \Lambda with a precision matrix. I took the liberty of changing the notation in GaussianFixedCov to denote the prior covariance of \mu as \Sigma_0.