[Feature request] Support schema auto-completion

[sizmailov]

Hi there!

Dynamic support for current rcsb scheme is cool, but it's not very helpful when you write scripts in IDE (and to a lesser degree in notebook).

It would nice to have auto-completion feature in "offline" mode. This would probably would require proliferation of the type system which is not pleasing to support. To avoid such tight coupling rcsbsearch with the current scheme the type information might be provided as a separate stub package (PEP 561) with no effect on runtime. Generation of stub files from schema should be rather straightforward.