Add properties @property/docstring to IStructure

[mkhorton]

Addendum to #3264, notifying @gpetretto, @janosh.

Two things I think we missed in #3264:

• A formal @property so that we get a docstring for the new IStructure.properties
• An edge-case for backwards compatibility. While I strongly recommend people do not use pickle as an archival format, nevertheless this is a very core pymatgen class, and I don't want to break people's workflows if they upgrade. Specifically, if you pickle a Structure from a previous pymatgen version, and then un-pickle it with the new pymatgen version, the __init__ will not be called, so the .properties instance variable is never set: this will then break code that relies upon it, like as_dict(). Luckily, adding the @property interface allows us to trivially catch this edge case.

[mkhorton]

Adding a note that, while reviewing this, I noticed a lot of methods on SiteCollection reference a "structure", even though these methods are also used by IMolecule/Molecule. Might cause some confusion.

[janosh]

Good idea! 👍