ENH Added Fisher's method and Stouffer's Z-score method

Closes #3092
you already have `len(pvalues)` 4 times, you could give it an name like `k` or n

```
z = (weights * zi).sum() / np.sqrt(np.sum(weights**2))
else:
z = zi.sum() / np.sqrt(k)
```

`asarray` is missing for this

looks readable to me

needs `raise` if not 1-D, because numbers would be wrong for 2d, or maybe it would raise at a uninformative point
could make it vectorized along axis=0, I guess