Implement getitem

It could be convenient to be able to write `A[:,3]` or `A[5,3]` when `A` is an abstract linear operator, if only for debugging purposes. Each costs one operator-vector product and could be implemented as

```
A[:,3] = A * e3
```

where `e3 = [0, 0, 1, 0, ..., 0]`. Similarly, `A[3,:] = (A.T * e3).T`. Finally,

```
A[5,3] = dot(e5, A * e3)
```
