Make the return of `traverse` optionnally type aware

[VEZY]

When we traverse the mtg (non-mutating version), we often already now what type of data our function will return. This could be given by the user as an argument so we could create a vector with this type instead of Any[].

[VEZY]

We could also get the type automatically in the same fashion that comprehension / broadcasting / mapping does. See this discussion: https://stackoverflow.com/questions/38285243/manual-type-inference-in-julia

But probably the best is to ask to the user, as they would know better.

[VEZY]

Note: update the functions that does use it, e.g. list_nodesor get_attributes.