The second commit in the big graph theory cleanup operation.
There is still quite some work to be done:
  1. RDF -> GIF transformation.
  2. Testing...

* [CODES_EXT,NEW] Added put_codes/[1,2].
* [DCG_GENERIC,NEW] Added dcg_arrow//[1,2].
* [DCG_GENERIC,NEW] Added dcg_idnent//1.
* [DCG_GENERIC,NEW] Added dcg_multi_list//2.
* [GRAPH_EXPORT,SC] Removed almost all code now.
* [GRAPH_EXPORT_GRAPHVIZ,SC] Rewrote this module to use DCGs now.
* [GRAPH_GENERIC,NEW] Relocated bipartite/4 and is_undirected/2, coming from UGRAPH_EXT.
* [GRAPH_GENERIC,FIX] Added meta- and RDF meta-statements for several predicates.
* [GRAPH_GENERIC,FIX] Fixed subgraph/2, anly the semidet/(+,+) version is generic enough for this module.
* [UGRAPH_EXPORT,EXT] Added more settings: encoding, color scheme, font size, overlap, vertex shape.
* [UGRAPH_EXPORT,SC] Split the code into code sections for graphs, edges, and vertices.
* [UGRAPH_EXPORT,SC] Rewrote ugraph_edge_name/2 to use DCGs.
* [UGRAPH_EXT,SC] Removed many predicates that are superfluous in the new situation and renamed many others.