Support nested JSON format for downstream formatting

[saraswat]

For knowledge representation and reasoning tasks, it is valuable to represent the information in a sentence in some kind of nested attribute value form. I am working on such a form in my fork. Here are some examples. The core idea is to nest forms, based on the first relation, and use xref/xlink fields to represent other linkages.

In general, the "KR" aspects of Props needs further development, in my opinion. (I see a lot of emphasis in the project on visualization.)

Please let me know if there is some interest in a pull request. (I will have to do some refactoring since I have also committed modifications necessary to run on MacOS, will need to pull them out.) Note: currently working to add information from features into the representation.

[gabrielStanovsky]

This looks awesome!
I'd be really interested in such a pull request.
Actually, the MacOS modifications part sounds very useful as well. Perhaps these can be merged in a separate pull request?

In fact, this direction of PropS for KR couldn't have come at a better time, we're actually looking at ways to do just that in a current project.

Would love to discuss this further.

Thanks!