robust fitting with student's t

So I have added the basic functionality. For now I have just added nu as an option to calc_lnprob in the FitterBase class. 

To use the option, you just set nu to a number greater than 2. If nu is very large, then you get back a fitting metric that is the same as a chi^2. If nu is just above 2, then your fitting metric has tails.  Some experimentation is needed, but maybe start with nu = 5. 

Eventually, one should include nu as a free parameter in the MCMC, However, right now the parameter list passed looks like it depends heavily on the Gaussian mixtures, so I am holding off on implementing this until I can understand how this works better.