Also, extract SingleGeometry and expose it it in its own sub-module

just saw you merged the other branch; cool :)
I'll work on this next.

@kif , I think that in order to make RingExtraction work for the calib guis, there'll have to be some larger refactoring of code.
Both are rather intertwined with the PeakPicker class, so I'll have to take that into account somehow. Perhaps I can add the option to add a PeakPicker instance instead of a SingleGeometry as a class attribute, so we can unify it all...
I also noticed that in the gui there is the option of choosing other methods for extraction (blob and watershed) so I'll work on including those options to RingExtraction too.
Very welcome to hearing ideas from you too, if you have them.
I'll push what I've done but mark as work in progress as I go so you can check it out and give input as well.