Add auxiliary function:

[math1um]

from: Loh, Po-Shen, and Jie Ma. "Diameter critical graphs." Journal of Combinatorial Theory, Series B 117 (2016): 34-58.

returned graph is guaranteed to be diameter-2-critical

compare Issues #631 and #632

add this in AUXILIARY FUNCTIONS

def diameter_2_critical_generator(g): #g and g_complement must be no more than diameter 2

if not g.is_connected() or g.diameter() > 2: return None gc = g.complement()

if not gc.is_connected() or gc.diameter() > 2:
    return None

h = g.disjoint_union(gc)
n = g.order()

for v in g.vertices():
    h.add_edge((0,v),(1,v))

h.relabel()
return h