Merge vidartf:ROIs

I tried to finish up an implementation, both of the GUI bit, and the application. To extend it, I guess the Signal class could have a property called 'slice_combination_function', or something similar, which could then be applied instead of np.mean. The mean function would then be the default one.

Also, I noticed Model.multifit will act strange if a size larger than 1x1 is used, should maybe look into how to avoid that (but at least nothing breaks when running it).