Added tests to test suite. Discovered bug in subgraph adding subvertex and subedge
Created tests for the placements module. Detected bugs
Removed superfluous code. Fixed bug in get_vertex_from_placement because it returned a placement instead of the vertex
Created tests for Routing Tables model
Added additional property for getting key mask combo
routing info now throws exception if the key mask combo is the same with different pre_subvertex, stores subedge_info in a dictionary keyed on key mask combo, get_subedge_info_by_key returns single entry not an iterable