Support for enhanced list comparisons

At some point we should try to support the grammar constructs listed in comparisons of list properties, e.g., list HAS ANY < 3, > 2.

One such correlated list query will come up in the CCPNC-DB implementation (tagging @jkshenton), where the following case might occur: if you have a flat list of chemical shifts of all species in a simulation, you will want to be able to do:

species_at_sites HAS Si:_ccpncdb_chemical_shifts HAS <= 3, >= 2

to request all Si chemical shifts between 2 and 3 inclusive. Of course it would be easier to keep the shifts in a dictionary keyed by element, at which case the query would be simplified to

_ccpnc_chemical_shifts.Si HAS <=3, >=2

but it would be great if we could support both.

Start by reading the OPTIMADE specification section linked in the issue, especially the comparisons of list properties grammar. Trace the repository's query parsing entry point to determine how correlated list queries are represented. Done means supporting both the nested species_at_sites form and the dictionary-keyed chemical-shifts form shown in the examples.