Break out algorithms in Graph.hpp into Algorithms/ folder

[nrkramer]

Graph.hpp has become pretty large and is a bit of a pain to sift through.

We should break-out Graph.hpp into two major modules:

1. Graph.hpp will contain the declaration of class Graph and define basic construction elements, edge, and node operations.
2. Graph/Algorithms/... will contain algorithm implementations that implement the algorithms declared in Graph.hpp.

If we want to remove the need for users to include algorithm files, we can create Graph_decl.hpp as a pure declaration file for class Graph, and have implementations in Graph.hpp and Algorithms/.... Tbh not entirely sure this is necessary, but it might avoid a circular include.

We can also use this as an opportunity to move some of the utility functions, such as reading and writing to various file formats, into Utils/Graph/... subfolder.

[nrkramer]

I can take this work

[ZigRazor]

Yes, I think is a good idea.
You can work on this!