Selection API for ferritin-core

[zachcp]

As of v0.1.0 the ferritin-core module has essentially two ways of selecting from an Atom collection - via the residue based iteration system and via the Selection system. These should be unified into a single common API. This discussion thread can be used to discuss the implementation and merits of different solutions

Refs:

• ferritin-init blog post.
• biotite
• molstar selectors