Optimization to the /src/graph_functions.cpp

[AntonisZks]

I believe that /src/graph_functions.cpp needs some optimization. Specifically we can clean up the file a bit from comments and some useless code. The GreedySearch algorothm needs a little imprevement by adding comments on the code, and make it compatible with the Graph data structure we already have. Also the function needs to have a better syntax since many STL structures are being used, in order to make it work correclty. So I suggest:

• [x] Clean up the file from comments.
• [x] Delete the Nodedata struct.
• [x] Make the GreedySearch compatible with the structures we already have.
• [x] Add documentation to the GreedySearch function.

@EvitaChasioti can you take care of that?

[StavrosJKw]

Deleted unimportant comments and used namespace std to make code more readable in GreedySearch()

[AntonisZks]

I organized my version of the Greedy Search and did the above. You can see the changes in my branch.