Add constatn to spectra

[ajasja]

Implement the addition (subtraction) of a constant and spectra.

spectra = spectra + 2  

Should add 2 to all y values. Subtraction should work similarly.

[ajasja]

Although it's easy to do s.dataset.y += 2. but I wonder if it is obvious enough (since all the other calculations are done with spectra directly. Also if we implement #53 then s.y += 2. is even shorter.

But of course s + 2 is the shortest and most elegant.