significant changes to the fitting package

Module Name	Original Location	New Location
physics_model.py	/fitting/model/	/fitting/
physics_peak.py	/fitting/model/	/fitting/
background.py	/fitting/model/background/	/fitting/
api.py	N/A	/fitting/

physics_model.py and physics_peak.py were moved to the root of the fitting package and an api was created that imports all relevant things from those two modules