Certain methods fail on immutable graphs

[jaanos]

The following methods fail for immutable graphs:

• GenericGraph.disjoint_routed_paths
• GenericGraph.genus
• GenericGraph.is_circular_planar
• GenericGraph.is_cut_edge
• GenericGraph.is_interval
• GenericGraph.layout_planar
• GenericGraph.multicommodity_flow
• GenericGraph.set_planar_positions
• GenericGraph.steiner_tree
• GenericGraph.to_simple
• GenericGraph.vertex_disjoint_paths
• Graph.gomory_hu_tree
• Graph.is_long_antihole_free
• Graph.is_long_hole_free
• Graph.is_weakly_chordal
• Graph.join
• Graph.seidel_switching
• Graph.topological_minor
• Graph.tutte_polynomial

I have changed these methods (or ones they depend on) to make a copy of the appropriate (sub)graph if one is needed. To prevent making unneeded copies of (potentially large) graphs, the methods to_directed, to_undirected, to_simple, subgraph, disjoint_union, union and join now take an additional parameter immutable specifying whether the returned graph should be mutable or not (with the default None specifying the old behaviour, i.e. keeping the mutability of the input).

A minor exception is the subgraph method, which would always return a mutable graph when the "add" algorithm was being used, but this is now made consistent with the "delete" algorithm and other methods.

The reason I need these methods to work on immutable graphs is that I am part of a team building a database of graphs with precomputed attributes (so the graphs being mutable would make no sense) which will be accessible through Sage - of course we still want to do stuff with the graphs that won't change them.