Add batch neighbor counting helper

Code from Scott.

```python
batch_mix_mat = np.array([[G.kernel[sample_idx==batch_i,:][:,sample_idx==batch_j].sum()
                 for batch_i in np.unique(sample_idx)]
                 for batch_j in np.unique(sample_idx)])
batch_mix_mat = batch_mix_mat / np.sum(batch_mix_mat, axis=1)[:,None]
batch_mix_mat```