This resolves ticket #100

This makes the code more efficient if the input parameter combinations
are much larger than the graph itself.

This allows them to be modified and passed forward in one expression.

This fixes a bug where some matches were skipped, due to a spurious,
abrupt termination of the search.

This also changes the default argument to 'random=False'.

This avoids copying property values when not necessary, if the index
remains the same after purging

After the removal of filtered vertices via Graph.purge_vertices(), the
values of known property maps are now kept consistent with the
unmodified graph.

This fixes ticket #92.

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

This fixes the edge invalidation check, when either the source of the
target vertex is removed.

This fixes ticket #97.

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

This fixes tickets #94 and #95