Rdf Collapsing Intro

[lnijhawan]

to run:

• doitlab
• run RDF_to_LPG_Example.ipynb

to run tests:

• pytests examples/tests

CHANGES:

• changed from rdflib_to_networkx_graph to rdflib_to_networkx_digraph
• more detailed description and specific examples of node collapsing via the new collapse_predicates function
• added a PredicateMultiselectApp which uses a combination of widgets to give a user the ability to select predicates they wish to collapse in an RDF graph
• developed CustomURIRef class, a way to store a URIRef URI but receive a 'pretty'/truncated version of the name for display purposes
• added pytests for both CustomURIRef class and the collapse_predicates function

[zwelz3]

In the PR description, I would make sure we are precise in the test procedure. Need to conda activate envs/develop, and the command is pytest. Everything else looks good. We can circle back on other improvements on the next ticket.