Add support for GraphViz cgraph library

[tobylorenz]

Hi,

GraphViz provides a native C API, called cgraph.
This adds supports for cgraph in a similar way to existing leda_graph.hpp and stanford_graph.hpp implementations.

The implementation already works well and fulfills all static BOOST_CONCEPT_ASSERT tests. However there are some issues remaining though. I tagged all issues in the source code with @todo.

1. The cgraph (Agraph_t) can be made directed or undirected, and also strict (=disallow parallel edges) or not. This is a runtime setting, set during instantiation or determined when loading graphviz files. I found no suitable way to handle this runtime setting already in the template in order to define the typedefs for bidirectional_tag and allow_parallel_edge_tag. How can this be handled?
2. cgraph has a longer list of properties on all levels, which I don't want to define all as property_tags. Is there a way to dynamically handle this and prevent BOOST_DEF_PROPERTY, property_map<...>, get, ... implementations for each of them?
3. The add_edge(u, v, ep, g) and add_vertex(vp, g) functions are currently configured to transport the edge's name in ep resp. the vertex's name in vp. But I could also feed in a std::map, so that add_vertex("V", g) becomes add_vertex{{"name", "V"}, g). Would that make sense?
4. The boost concept checks fail, unless I pull all functions into global namespace by using boost::out_edges;, .... Any idea why this happens?
5. subgraphs are currently not implemented.

But overall it already works quite well for me. I can copy an existing graph into the cgraph structure to get it visualized with GraphViz.

Bye
Tobias