Added support for an finish_tree_edge event in depth_first_search.
Added SCC-Visitor support for strong_components.

Rewrite of the named_parameter-interface to use same facilities as
depth_first_search. Removed old "by-hand-dispatching".
Added visitor for recording successors.
Merge branch 'feature/strong-component-visitor' into develop
Merge branch 'feature/finish_tree_edge-in-dfs' into develop