Week 2

[pranavtbhat]

• [x] Expand graph interface with mutators, constructors, etc.
• [x] Make graph types mutable. (Mutation becomes too expensive when the graph is immutable)
• [x] AdjacencyModule Interface
• [x] LightGraphsAM
• [x] SparseMatrixAM Interface
• [x] Property Module Interface
• [x] DictPM
• [x] NDSparsePM
• [x] NullModule
• [x] Tests
• [ ] Update Readme

@shashi @ViralBShah