adding STIX2 Bundle to MemorySource

[elegantmoose]

https://github.com/oasis-open/cti-python-stix2/blob/master/stix2/datastore/memory.py#L36

Cant remember what I (or we) initially thought about adding a STIX2 Bundle to MemorySource but after using API, I expected the MemorySource to read in the parsed JSON STIX2 bundle and have access to the objects. Instead it just had access to the bundle object for querying. Also - we grab the objects out of a bundle if it is a dict bundle, but not if it is a python-stix2 bundle - which seems incongruous.

[gtback]

Could probably combine those first two if clauses into just if isinstance(stix_data, collections.abc.Mapping): or if isinstance(stix_data, (_STIXBase, dict)):

docs for collections.abc.Mapping, which _STIXBase subclasses.

[gtback]

I'm putting this in the STIX 2.1 milestone for now. If we want to release it in the 1.0.x branch, we can cherry-pick the necessary part of @emmanvg's commit.