Update `AtomArrayPlus` to natively support entry-level data

[nscorley]

It would be convenient to use AtomArrayPlus as a unified molecular representation without needing to be concerned with passing around a dictionary.

For example, upon loading a structure with AtomWorks, one could store information such as the resolution, pH, etc. directly within the AtomArrayPlus; annotations may then also be provided at the entry (and perhaps later, chain-) level.

These entry-level annotations should only support basic types and should be CIF-writable