Add ontology support

[ajtritt]

Originally reported by: Andrew Tritt (Bitbucket: ajtritt, GitHub: ajtritt)

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Spec objects should support adding ontology and ontology id

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• Bitbucket: https://bitbucket.org/lblneuro/pynwb/issue/15

[oruebel]

Related issues:

• NWB Schema: https://github.com/NeurodataWithoutBorders/nwb-schema/issues/1
• PyNWB issue with discussion from previous hackathon: https://github.com/NeurodataWithoutBorders/pynwb/issues/513
• Related use case: https://github.com/NeurodataWithoutBorders/nwb-schema/issues/477

A solution for this is currently being developed in HDMF:

• https://hdmf.readthedocs.io/en/stable/tutorials/External_Resources.html#sphx-glr-tutorials-external-resources-py

[bendichter]

@rly , @oruebel I'm thinking about creating a tutorial in PyNWB to illustrate best practices in using ndx-external-resources. What do you think?