Renamed little names.
Inlined the definition for the exception subclass NetworkNotConnected.
Move the GasEdge in the folder dedicated to edge hierarchy.
Renamed the GasEdge into GasEdgeTopological.
Separate the logical informations from the physical one for the edge
hierarchy.
[SOLUTION DOES NOT COMPILE] Refactor the DotRepresentationValidator in
order to use the new paradigm of ``faceted'' edges and nodes.
Fixed the specification files using the right GasNode variant. Still
many tests do not pass.