Summing dimension mismatch

In `cliffordlayers/nn/functional/groupnorm.py` on line 211:

```
running_cov_orig.copy_(running_cov.view(I, I, B, int(C / num_groups)).mean(1, keepdim=False))
```

I get a tensor size mismatch error. I think the dimension 1 is wrong and it should be 2:

```
running_cov_orig.copy_(running_cov.view(I, I, B, int(C / num_groups)).mean(2, keepdim=False))
```