Allows from_numpy_matrix to create parallel edges

Adds the `parallel` keyword argument to the from_numpy_matrix()
function. If the adjacency matrix has integer entries, this is set to
`True`, and the `create_using` keyword argument is an instance of
`MultiGraph`, then each positive integer entry in the adjacency matrix
is interpreted as the number of parallel edges, each with weight 1,
joining two vertices. Otherwise, if any of these conditions is not met,
the function behaves as it previously did: an integer entry is
interpreted as the weight of a single edge joining the two vertices.

This is a backwards compatible change.