Spectra -> internal representation

For now a Spectra is simply a list of Spectrum. Spectrum has the x, y information regarding is position. From this there is several missing information / optimization to bring:

• add offset information
• it would be nice to have a True Spectra class that simplify the link between Spectra and Spectrum and that could be given to other processing. It could contain Mu, NormalizedMu... For now we have to parse all Spectrum to retrieve those information. But this is not trivial in the way now Larch and Pymca are treating directly those Spectrum and are not aware about the Spectra. Might be time-consuming.
• parameters as normalized energy... are computed and stored for each spectrum. Which is a waste of time and memory, maybe (certainly) there is something to optimized that. Come again with the treatment on a Spectrum and not a Spectra by libraries (larch)