Shift `from_set` attribute to attribute of S3 "set" vector class

[cynthiahqy]

• Want a vector class that extends the to & from columns in the edge list;
• Possible names: xmap nomenclature set or xmap nomenclature layer;
• would be an extension on factor vector; with levels being the "unique set"

[cynthiahqy]

Discussed with rob and Emi, just use factors for now (levels() returns unique set)