Updates for spectroscopic data reductions

Updates to add iterative reweighting least squares to make the fitting m..ore robust to outliers. Added a gaussian process fit for the sensitivity function. Added fitting the full cross-correlation option to interidentify.
I'd prefer to leave fit the way it is, but can you add this to interfit?   I'd just replace the self.fit() in interfit with what you have here.  Then, there is also not the hard coded iterations as well.