Fisher scoring now works for both the constnat and the isotropic model.  However, I need to go in and make sure I am doing absolutely everything correctly in the 2nd derivatives.  The whole setup breaks when given a rician sigma of 10, and small parameters otherwise (1 & 2).  This makes some sense, since the model here is going to be giving signals that are somewhere around exp(1) * exp(-2ish * 2), with a variance of 10 added to two channels...