Get Neighbors implementation for Undirected and Directed Graphs with Tests

[rohanbadlani]

Throughout the usage of SNAP platform, there are many graph algorithms for which the user might need neighborhood data and hence adding that functionality as easy to use methods.

[Build] The build is passing.

[Testing] Tested the functionality and provides results as expected but will be writing rigorous tests.

I am not sure on where to add the documentation for this, but I have used comments in the codebase to make things quite clear and will be happy to help provide/edit documentation for this.

SUNet Id: rbadlani.

[rohanbadlani]

Adding in the implementation for directed graphs as well.

[rohanbadlani]

Added Tests in the tests/tests-TUNGraph.cpp and tests/tests-TNGraph.cpp that follow the same convention as all the other tests. These test basic functionality using a small test graph.

SUNet Id: rbadlani