This fixes compilation on boost versions < 1.60.

Fixes: #410

This removes an unnecessary reference to
<boost/math/special_functions/relative_difference.hpp>.

Resolves: #409

This fixes issue #405

This adds the parameter "return_reached" so that the user can provide
and reset the initialization for "dist_map" and "pred_map", to improve
the performance of multiple searches that do not visit the whole graph.