Fix Floyd-Warshall when weights have 0 weight. Addresses #976.
Clarify comment in code regarding nan weights.
Make `nonedge` default to 0.0. Add more comments.
Clarify description of `weight` parameter.
Allow 'weight' to be callable.
For example, weight=lambda attrs: attrs.get('weight', 0).
Handle nonzero edges via masked arrays.
Also, allow custom multigraph_weight opertor functions.
Revert commits 1bca2d1 and 31b5b70.
Merge pull request #976 from chebee7i/fw_bug

Subtle bug in floyd_warshall_numpy