json-graph-specification

Several updates: (a) update JSON Schema to Draft7, (b) add 'id' and 'title' fields to schema definition (c) change nodes array into keyed objects, (d) put 'id' field back into graph object, (e) simplify 'graphs' array, (f) remove 'null' from properties objects

Update tests for new schema

Swap json-schema for json_schemer b/c the former is no longer maintained and does not support JSON Schema Draft7. Add support for Draft7.