Overhaul the system normalizer

• required by optimade api #205 (closed)
• related to missing geometry metric #204 (closed)

The current system normalizer is only run for one system. It is ok to only perform the costly system and symmetry classification for a few systems, but some things should be normalized for all systems:

• atoms
• species
• hill, full, reduced
• lattice, positions, PBC

Furthermore, the normalizer and metainfo does really account for

• the difference between sites and elements, i.e. atoms and species
• different versions of the same element in different sites (species?), e.g. Na and Na2

Some things are missing:

• nelements vs. nsites
• element concentrations