BGL Depth First Search

[colin-lewis-19]

Adds a solver that utilizes BGL depth first search

Depends on #76 and #80

[marip8]

@colin-lewis-19 per our discussion, I made the following changes:

• Added comments to the BGL solvers to document that some event visitors are created implicitly in the search and need not be specified via template parameter by the user
• Moved the creation of the final DFS event visitor required by the search into the search methods for each solver variant
• Opened BGL: Revise cost_recorder event visitor #82 to document revisions required for the cost_recorder event visitor

Any other changes need to be made?

[marip8]

Turns out there were a few unresolved issues, but I fixed them and was able to produce these graphs for the DFS search:

DFS with null visitor (i.e. without early search termination)

Note that the distances of these nodes from the source are not optimal because DFS is not guaranteed to find the optimal solution

DFS with early search termination

Other changes

I also removed the Visitors template from the solver partial implementations (BGLSolverBase, BGLSolverBaseSVSE, BGLSolverBaseSVDE) because:

• They do not require the visitors for any of their tasks
• Including the visitor template parameter made things more complicated downstream
• Including the visitor template parameter essentially prevents solvers from being cast into one these partial implementations without knowledge of the visitor template type (for the purposes of serializing a Graphviz file, for example)