Graph.deleteVertex(): Remove edges from other adjacency lists. Graph.deleteEdge(): adds default None to parameter id. Graph.__str__(): works when graph is empty Graph.dfs(): added new parameter to specify partial dfs Graph.vertices_topological_sort(): added new parameter to specify partial topological sort
Graph.deleteVertex(): Remove edges from other adjacency lists.
Graph.deleteEdge(): adds default None to parameter id.
Graph.__str__(): works when graph is empty
Graph.dfs(): added new parameter to specify partial dfs
Graph.vertices_topological_sort(): added new parameter to specify partial topological sort

Added unit tests to exercise changes and new functionality.

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