Reduce the spectra metainfo complexity
The spectra metainfo (used by eels and xps parser, on branch experiment
, 205aab5a) in nomad.datamodel.metainfo.common_experimental
defines too many details. We should try a different approach. It is basically always a set of channels (counts, energies, etc.), which are arrays with the same number of values. Currently, it defines a quantity for all possible channels (probably not exhaustive). We should reduce this to the most common channels (count, energy) and provide a higher dimensional quantity to store all other channels. The description for those other channels (name, id, unit) could be stored in a repeating sub section.