Implemented subedge and subgraph
Created test skeleton for subgraph
Added implementation for create_subvertex method in Vertex
typo in exception in subvertex
Implemented create_subedge in Edge
Completed tests for the graph model
Modified references to vertex/edge/graph to be optional. Modified variable names so they are more explicit. Needs testing
Updated Graph Model tests to accomodate for changes with graphs and subgraphs
Added supporting methods for returning subvertices for a given vertex, vertex given a subvertex, subedges given an edge and edge given a subedge
Forgot to update this constructor call
Merge remote-tracking branch 'origin/master'