This closes #222 and #223.

Disable conversion of 'zero' and 'infinity' values in search functions when the distance type is 'python::object'

This changes the way search visitors are wrapped, such that the original
class remains unmodified. With the previous code, the original classes
would be repeatedly modified, adding unnecessary overhead, and even
possibly causing the number of nested calls to exceed the maximum
threshold.

Fix problem in search functions when 'zero' and 'infinity' values cannot be cleanly converted to property type

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.