Graphical degree sequence

Test sequences for graphiness.

is_graphical(sequence[, method])	Returns True if sequence is a valid degree sequence.
is_digraphical(in_sequence, out_sequence)	Returns True if some directed graph can realize the in- and out-degree sequences.
is_multigraphical(sequence)	Returns True if some multigraph can realize the sequence.
is_pseudographical(sequence)	Returns True if some pseudograph can realize the sequence.
is_valid_degree_sequence_havel_hakimi(...)	Returns True if deg_sequence can be realized by a simple graph.
is_valid_degree_sequence_erdos_gallai(...)	Returns True if deg_sequence can be realized by a simple graph.