Fix #18 and #5, exception when partitioning net

Usage of nx.average_shortest_path_length dont allow a partitioned
net, and have to be executed once for each subgraph on the graph

command.py

@@ -82,11 +82,20 @@ class MetricShell(Cmd):
             effects = self.simulation.get_effects()
             multinodes = [x for x in list(effects.keys()) if len(list(effects[x].keys())) >= 5]
             multimulti = [x for x in list(effects.keys()) if len(list(effects[x].keys())) >= 20]
-            difflen = nx.average_shortest_path_length(self.simulation.graph)\
-                    - nx.average_shortest_path_length(self.model.G)
 
-            components = list(self.model.G.subgraph(c) for c in nx.connected_components(
+            # Get all connected subgraphs of the current graph
+            components = list(self.simulation.graph.subgraph(c) for c in nx.connected_components(
+                                                                  self.simulation.graph.to_undirected()))
+
+            basecomponents = list(self.model.graph.subgraph(c) for c in nx.connected_components(
                                                                   self.model.graph.to_undirected()))
+
+            # Average SPF is only able to be computed on connected nodes, if one node is disconnected 
+            # it will fail computation. This code executes Averate calculation once pr. subgraph
+            # to circumvent this limitation
+            difflen = sum([nx.average_shortest_path_length(x) for x in components]) \
+                    - sum([nx.average_shortest_path_length(x) for x in basecomponents])
+
             cnodes = 0
             if len(components) > 1:
                cnodes = sum([len(g.nodes()) for g in components[1:]])