BGL Graph Serialization

[marip8]

This PR adds the ability to serialize a boost graph to a .dot file for visualization of BGL implementations of the solver

[marip8]

@colin-lewis-19 can you review this as well?

[colin-lewis-19]

This looks good to me, and worked as expected when I ran it.
I don't feel like the implementation files are so large that we need to be splitting one class across multiple .hpp's, since that can hurt readability.

Edit: The bgl_ladder_graph_solver is going to continue to grow though and I am not opposed to getting ahead of a refactor.

_{The rest of descartes_light uses header guards instead of #pragma once though}

[marip8]

I don't feel like the implementation files are so large that we need to be splitting one class across multiple .hpp's, since that can hurt readability.

Good point. I think now that we have the custom vertex class, the graph serialization can be a stand-alone function. I'll move them out so they're separate and can be used by other BGL implementations (provided those implementations use the same graph typedefs)