[ScanFit/SimpleFitGui] fit background only about pymca

[vasole]

• The only way to just fit a background in the "Simple Fit" is to put such a high number as sensitivity that no peak is found. Alternatively, you can set the constraints of the peak found (if any) to "IGNORE"
• For the customized fit, you can select "User estimated Constant" or "User estimated First Order Polynomial" among the background functions.

[vasole]

I agree I should either allow the option None to the simple fit function or to propose the background functions as fitting functions.