Returns True if some multigraph can realize the sequence.
Parameters: deg_sequence (list) – A list of integers Returns: valid – True if deg_sequence is a multigraphic degree sequence and False if not. Return type: bool
The worst-case run time is O(n) where n is the length of the sequence.
 S. L. Hakimi. “On the realizability of a set of integers as degrees of the vertices of a linear graph”, J. SIAM, 10, pp. 496-506 (1962).