Consider making core construct containers keyable

[bworrell]

For consideration:

Have top-level containers like ttps, indicators, incidents. etc., keyable, so that we can interact with them like dictionaries. This may make use of relationships a bit easier.

So we could do something like this:

ttp_idref = related_ttp.idref
ttp_definition = stix_package.ttps[ttp_idref]

[gtback]

I like it! It might take a bit of Python magic, but that's never stopped me/us before!