add a function to split graphes
def get_graph_distance(self, graph1, graph2) added:
    return the distance between two graphes. The distance is the smallest
    distance between two nodes of each graph
improve clean_graph
improve clean_graph: the function works :-)
add an option in plotgraph to plot the shortest path
fruchterman_reingold algorithm added
fruchterman_reingold algorithm improved
fruchterman_reingold algorithm improved. Add an argument to keep some vertices fixed
option to plot nodes in plot_graph added
keep the adjacency matrix boolean for fruchterman_reingold algorithm
compute the area of the graphe to scale spring constant in fruchterman_reingold algorithm