IsMutable trait

#88, #89, #85 raise the need to have a specific interface for mutable graph.
This is useful to explicit the output type of the graph for function returning another graph. As a mutable graph could model any graph structure, the output of these functions would just be a graph of the same type as the input graph. Would fall under the IsMutable trait the graphs implementing [add|remove]_vertex!, [add|remove]_edge! and some non mandatory ones like zero, [add|remove]_vertices!, [add|remove]_edges! (zero could eventually be mandatory).

Maybe that for non mutable graph, we could add a method which would precise the expected return type of the graph under mutation.

There are still some corner cases like functions taking as input an undirected graph and returning a directed one, for which this trait will not help to get it generic.

Start by reading issues #88, #89, and #85 to understand the need for an IsMutable graph interface. Define the supported mutation methods and expected graph return-type behavior, including the unresolved directed-versus-undirected corner case; done means the interface scope and generic behavior are agreed and documented.