Now paths are weighted according to their trust value until the last
hop, and paths are not counted twice.

Thins changes the graph filtering code slightly to wrap graph types with
GraphWrap, which automatically updates the edge index list when edges
are removed and added to the graph.

This also changes how graphs are passed to algorithms, which is now by
reference instead of pointer. (hence this touches lots of code, but
changes are trivial)

This adds the centrality module, which supports betweenness, eigentrust,
pagerank and absolute trust.