Plot several spectra using the same navigation

I added a function which makes it easier to compare spectra with the same dimensions (as discussed in https://groups.google.com/forum/#!topic/hyperspy-users/WLfOjzE6bhk ).

Useful for plotting data acquired at the same time, for example low loss, core loss and EDX.

(I'll add some documentation in the user guide to this pull request a little later).