Lmfit model for xrf

The fitting model for xrf is included. It is still in process, and some file structure might need to be re-organized.
That feels like it is something that should be done through the constraint system in lmfit which (as I understood it) could be used to constrain the value of area of the first peak to directly control the area of the other peaks. 

cc @danielballan (even though you should be writing :smiling_imp: )