Remove Nodes and use Integers instead

[ClaudMor]

Node objects could be useless.

We may replace them with Integers (of type N <: Integer, to be added as the third AbstractMultilayerGraphs parametric type). This would imply:

1. Substituting the node::Node field of MutlilayerVertex to a node::N (where {N <: Integer});
2. Reimplementing rem_node!(mg::AbstractMultilayerGraph{T,U,N}, n::N) where {T, U, N} so that it updates the node field of the MultilayerVertexs stored in v_V_associations that have node > n.

Replacing Nodes with Integers would have the following benefits:

1. Simpler code throughout the codebase except for rem_node!;
2. Better performance of fadjlist and badjlist.

And the disadvantages hereafter:

1. Need to keep and maintain an external mapping to associate integer nodes to their "meaning" (i.e. the agent they may represent if one uses an AbstractMultilayerGraph as an argument to Agents.jl's GraphSpace, see Improve integration with Agents.jl #61.
2. ???