linear regression example / docs
I'd like to use the linear algebra stuff in Arraymancer to do linear regression. In python/numpy, I can do this as, e.g.:

```Python
np.linalg.inv(X.T.dot(X)).dot(X.T).dot(y)
```

I didn't see a matrix inversion proc in the docs or via git grep. Is this outside the scope of `Arraymancer`, or am I missing something, or other? thanks!