Implementing Issue #15 (changing edges_mustexist and edges_mustnotexist to required_edges and prohibited_edges).
Implementing Issue #15 (changing edges_mustexist and edges_mustnotexist to required_edges and prohibited_edges).

git-svn-id: e6d44ec3d56a4b51a9cad284a9139aeaee5a4315@96 3447e8dd-9f38-0410-a211-e3037a7e05e3

arnaudsj/pebl,arnaudsj/pebl,arnaudsj/pebl