Check that isequal is implemented for nodes

[VEZY]

We now check that a node is in a set during graph traversal. I didn't check if we have a proper implementation of == of two nodes. If so, it is also interesting to implement a method for hash.

[VEZY]

See https://github.com/JuliaServices/AutoHashEquals.jl for an automatic way of doing this.

[Samuel-amap]

Note : there are functions for testing Node and AbstractNodeMTG equality (see in https://github.com/VEZY/MultiScaleTreeGraph.jl/blob/master/src/compute_MTG/equality.jl#L6 ).