Visitor Templates for BGL Solver

[marip8]

This PR adds a template parameter to the BGL solver classes for generically specifying the event visitors to use in the search. Since the event visitors themselves do not inherit from a single base class, they can only be specified and stored within the search class using an additional template parameter. This makes the instantiation of the BGL search classes more difficult, but prevents the need for creating separate class instances for every useful combination of event visitors that a search might need.

In order to make the BGL search classes a little more user-friendly, the BGLDijkstraSVSESolverD/F and BGLDijkstraSVDESolverD/F typedefs represent the configuration of the "most effective" solvers (currently using the early terminator event visitor). Additionally a few macros were created for explicitly instantiating template classes for a Cartesian product of template parameters (i.e. float types with event visitor types), so the instantiation of each solver with every combination of float type with event visitor does not have to be manually specified.

Addresses #79

[marip8]

@colin-lewis-19 please review

[colin-lewis-19]

Looks good. The templating & macros are a bit dense, but I think the increase in usability is well worth it