This includes user-defined types, as well as vector types and strings.
This affects dijkstra_search(), astar_search() and bellman_ford_search().

This also makes all Edge classes inherit from the same base class of the
same name.

This fixes tickets #94 and #95

Images are now converted to png a posteriori for html output, but are
used as pdf for latex.

This fixes a leftover bug from the removal of the 'core' submodule.

This module contains several search algorithms and a visitor interface.