DOC: Fixes #348. Improve the EELS chapter.

* Add the align_zero_loss_peak method to the User Guide.
* Other small improvements.
Merge pull request #360 from francisco-dlp/DOC_improve_eels_chapter

DOC: Fixes #348. Improve the EELS chapter.
Merge branch '0.7.x' of https://github.com/hyperspy/hyperspy into 0.7.x