Vectorize methods

I think all of these functions could accept 3D arrays of shape `(n_voters, n_cands, n_elections)` or `(n_elections, n_voters, n_cands)`, whichever makes more sense, and return a list of winners for each election.

```
np.array((impartial_culture(5, 4), impartial_culture(5, 4)))
Out[16]:
array([[[1, 2, 0, 3],
[2, 1, 3, 0],
[3, 0, 1, 2],
[3, 1, 2, 0],
[2, 3, 0, 1]],

[[0, 2, 3, 1],
[2, 3, 1, 0],
[1, 3, 2, 0],
[2, 1, 0, 3],
[0, 2, 3, 1]]], dtype=uint8)

_.shape
Out[17]: (2, 5, 4)
```