Add 2 graph properties: is_diameter_critical, and is_diameter_2_critical

[math1um]

Uses make_diameter_critical() function from Issue #631

IDEA: take a connected input graph, find a diameter-critical subgraph on the same vertex set

if the number of edges doesn't change then the subgraph is the graph

and our input graph must be diameter critical

def is_diameter_critical(g): #True if graph g is connected and diameter critical

if not g.is_connected():
    print("in is_diameter_critical: input graph is not connected, returning None")
    return None

h = make_diameter_critical(g)

if h.size() == g.size():
    return True
else:
    return False

def is_diameter_2_critical(g):

if not g.is_connected():
    print("in is_diameter_2_critical: input graph is not connected, returning None")
    return None

if g.diameter() == 2 and is_diameter_critical(g):
    return True
else:
    return False