Unsemantic datamodel

[jesper-friis]

The species_at_sites property of OPTIMADEStructureAttribute is currently a list of strings, referring to the name of the individual species.

Having species_at_sites being a ref-type to OPTIMADEStructureSpecies would be more semantic and less entangled. This would also be a more efficient way to store the data.

[CasperWA]

That's a fair observation. Again, these data models are taken as verbatim as possible from the OPTIMADE specification, with the limits that this entails as well, of course. See the section on species_at_sites.

I'm sure there are multiple improvements to be made in the data models here.
For example, one could also compact the attributes, combining them with the top-level id and type, eliminating the need for an attributes reference and the OPTIMADEStructureAttributes model (it would effectively be combined with the OPTIMADEStructure model.