add a version almost completed of mapback while fixing cnograph.
add a **in** operator and small method to split ands reactions into its ORs
- Update Link class with names for the + and - links
- Remove the compress_ands option (we never compress them)
- Revisit the attr_dict in add_edge, add_node. This is not an easy task and will probably
  need other iterations but this is a progress compared to the previous version.
  attributes can for instance be passed to add_reaction() method now.
- add method called is_reaction_in to check if a reactions is present in the grpah
- add_edges_from is now calling add_edge
- add _get_and_attribute method
- add _update_and_gate_names to fix https://github.com/cellnopt/cellnopt/issues/38
- add  signed_predecessors method
- add set_node_attributes method
- add set_edge_visibility_from_reactions method
- add edge2reaction method
- remove_edges_from now calls remove_edge method
- rename_nodes renamed into relabel_nodes (to be similar to the networkx syntax)
- rename_node/relabel_nodes was updated/simplified
fixing a couple of regression bu/typo from the previous commit
update tests
move poisson methods from cnograph to xcnofraph