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's why xrf fitting is complicated. The real case is that, sometimes you need to set ratio as a fixed value following the constrain, but sometimes you also need to set the ratio as a fitting parameter to  join in the fit. That's why I create another function to wrap gaussian so ratio can be an input.