Now we can compare spectra with each other by subtracting saved from the current one
Some small fixes - now user cannot subtract the line from itself
Fixed bug with quiz() function: crash if user hits "Cancel" button
Power limits changed to (0, 4), so now it is not necessary to count zeroes in labels
Now the main line could be made invisible with "v" or "V" key
And "V" key
Merge branch 'feature/comparison' into development
Bug fix in speview ("V" key). Changed version and docs