Spectra normalizer

Following #1316 (closed) I will develop and push a SpectraNormalizer and the visualization inside ElectronicPropertiesCard. This should:

• Normalize spectra to the max intensity.
• Define type for all types of spectra (EELS, XAS, XANES, EXAFS, XPS...).
• Merge EELS data into Spectra.
• Create visualization in between BZ/BandGap and GreensFunctions cards or in its own card SpectroscopyProperties
• Add tests for Spectra.

@lucamghi @sanbrock I think at this point you should be informed of my ovjective. This will only affect Area C data, but it might be nice to prepare the changes such that, when Area B wants to tackle this, what I did it is not a complete mess or too different than expected, and that it is easy to extend with extra metainfo 🙂

I will push a branch soon (a couple of weeks max, if the reprocessing does not show too many errors this week).