Include more evidence information in CX2 output beyond ECO term

[pkalita-lbl]

Currently when edges are added to CX2 networks we add an evidence attribute to the edge. In the underlying GO-CAM model the edge represents an item from an Activity's causal_associations. That CausalAssociation has a list of EvidenceItems that we iterate over and pull out the term to produce a list of strings for the edge annotation:

gocam-py/src/gocam/translation/cx2/main.py

Lines 203 to 204 in 29b703a

	if association.evidence:
	edge_attributes["evidence"] = [e.term for e in association.evidence]

But those EvidenceItem instances have more than just a term that could be helpful to show. The question is how to translate the EvidenceItem instances into the basic types that CX2 supports.

[pkalita-lbl]

One suggestion from the NDEx team was to use an HTML string as an edge attribute. Since the HTML will be rendered, you can arbitrarily condense complex information into a single string.