Implementation of Some Algorithms for the Densest Subgraph Problem

[Elfarouk Harb]

Hi Everyone, 

My name is Farouk, I'm a first time contributor. 

I've sent in a PR that implements two popular algorithms for solving the Densest Subgraph Problem (DSG), which is a foundational problem in graph theory with a wide range of applications. Dense subgraphs often provide insights into network structures, such as community detection, and can serve as a key subroutine in unsupervised clustering techniques.

I've implemented two popular (and simple) approximation algorithms, and was hoping for someone to review/have a look. 

Thanks,

Farouk