Add a new abstract class in order to make Nodes and Edges for Netwon
Raphson system an abstract data type: this has been necessary because
we can manage the summary table output for networks such that contain
two nodes u,v linked with two edges a,e such that a=(u,v) and e=(u,v).
Without this introduction we have to use the identifier, which in
the above case is the same for both edges a and e, resulting in a conflict
and considering only one edge instead of two different edges.