Random walk groupby and scipy sparse indexing

[brianhie]

1. Random walk only works with groupby, fix to allow for a random walk on the complete graph: https://github.com/brianhie/evolocity/blob/07d8ea1cc191dafd16479b2e409d7654f8d8a1d2/evolocity/tools/random_walk.py#L90
2. Scipy sparse indexing is broken, perhaps due to updates in underlying package but still investigating: https://github.com/brianhie/evolocity/blob/07d8ea1cc191dafd16479b2e409d7654f8d8a1d2/evolocity/tools/random_walk.py#L106