Add Vector::axcpy method
Add Vector::axcpy method

The added method `Vector::axcpy` generalises `Vector::gemv` to
noncommutative cases since it allows us to write for `gemv`
`self.axcpy(alpha, &col2, val, beta)`, instead the usual
`self.axpy(alpha * val, &col2, beta)`. Hence, `axcpy` preserves the
order of scalar multiplication which is important for applications where
commutativity is not guaranteed (e.g., matrices of quaternions, etc.).

This commmit also removes helpers `array_axpy` and `array_ax`, and
replaces them with `array_axcpy` and `array_axc` respectively, which
like above preserve the order of scalar multiplication.

Finally, `Vector::axpy` is preserved, however, now expressed in terms of
`Vector::axcpy` like so:

```
self.axcpy(alpha * val, &col2, beta)
```

sebcrozet/nalgebra,sebcrozet/nalgebra