significant changes to the fitting package
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================ =========================== ==============
 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