Implement difference of spectra and STOCSY

[lpatiny]

We should be able to set a reference spectrum and all the other one will be displayed as a difference.

I'm not sure what is the best approach to achieve this. It should be in the Matrix panel because the peaks have to be normalized at the level of the X axis.

One way is to have a select drop down menu in the Matrix panel in order to select the reference spectrum and add a new filter that would sub