Make the diameter() return None when the graph is not connected
When a graph is not connected, its diameter doesn't make much sense. Although, when the diameter function is used on a non-connected graph, it returns a value. We should change diameter () (in register.ml), so that it checks if the graph is connected before using the diameter function. If the graph is not connected, it should return None, and if the graph is connected, it should return Some d (with d being the diameter).