Alias 'RowDVectorView<>' is missing for the result of DMatrix<T>.row()

Consider this:

```rust
let my_dmatrix: DMatrix = ...;
let x: DVectorView = my_dmatrix.column(0); // there is the DVectorView alias, happy days
let x: RowDVectorView = my_dmatrix.row(0); // but RowDVectorView doesn't exist
// ^^^^^^^^^^^^^^
```

Seems like a type called `RowDVectorView` might be missing for consistency?
```rust
pub type RowDVectorView<'a, T, RStride = U1, CStride = Dyn> = Matrix>;
```