Remove some redundancy in the imports
Resolves some basic issues with fitting multiple models sets (for linear fits only, which is the only place that is supported for now).  Right now this only supports fitting with *one* input across all models (such as when evaluating the model with model_set_axis=False).  This is actually in line with what was previously supported.  The only major difference is that it's no longer to transpose the 'y' input before passing to the fitter.  This is, as I see it, an internal detail of how the fitting algorithm works and not something users should be responsible for.  I'll clean this up further once I've merged this work #2580, which will make further improvements easier.