ENH: simple implementation for correcting energy shift in EELS line scan

I made a simple function for correcting energy shift by using the zero loss peak, without altering the original data through interpolation. However the spectra is saved as individual spectrums in an array, making it a non-ideal way for continued processing.

So making a pull request for discussion on how to best get this to work (https://github.com/hyperspy/hyperspy/issues/118).