Dos normalization menu even when normalization is not possible

@himanel1 @ndaelman I realized the kebab menu in the DOS always appears and let user click on "Normalize intensities", independently of whether the normalization factor was extracted by the DosNormalizer or not.

Could this be rendered or not depending on that condition? Otherwise is confusing that the units on the DOS axis change from "states / eV" to "1 / eV" but nothing happens.