Extension and reimplementation of Shirley background substraction

[MarcBHahn]

Hi I reimplemented the section to substract a Shirley background as used in XPS. It is now possible to substract a Shirley background only within the active region as it is often done in XPS. Additionally the implementation seems to be more stable when working with data having a bad SNR. Best wishes Marc

[wojdyr]

Hi Marc, that's a much needed feature. I searched my e-mail archive and I found a couple requests for selection of end points for Shirley background - the first one probably from 2013. I have a couple of other pull requests waiting (for fityk and xylib). I'll get to it soon.